diff --git a/AGENTS.md b/AGENTS.md
index 9080c69..6317527 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -104,7 +104,7 @@ return M
 - No trailing whitespace
 
 ### Comments
-**DO NOT ADD COMMENTS** in code unless explicitly requested by the user.
+Add comments to explain non-obvious code sections, complex logic, and public API functions. Keep comments concise and relevant.
 
 ### Parser Module API
 All parser modules must implement:
diff --git a/lua/scl/builtin_instructions.lua b/lua/scl/builtin_instructions.lua
index 5059ef8..a9d9066 100644
--- a/lua/scl/builtin_instructions.lua
+++ b/lua/scl/builtin_instructions.lua
@@ -1,5 +1,9 @@
+-- TIA Portal built-in instructions for SCL Neovim plugin
+-- Defines functions, function blocks, and their parameters for completion
+
 local M = {}
 
+-- Built-in functions (return values, no state)
 M.FUNCTIONS = {
   ADD = true,
   SUB = true,
@@ -38,6 +42,7 @@ M.FUNCTIONS = {
   I_BCD = true,
 }
 
+-- Built-in function blocks (have state, need instance)
 M.FUNCTION_BLOCKS = {
   TON = true,
   TOF = true,
@@ -64,6 +69,8 @@ M.FUNCTION_BLOCKS = {
   DATALOG = true,
 }
 
+-- Parameter definitions for built-in instructions
+-- Format: { inputs = { { name, type }, ... }, outputs = { { name, type }, ... } }
 M.PARAMETERS = {
   TON = { inputs = { { name = "IN", type = "BOOL" }, { name = "PT", type = "TIME" } }, outputs = { { name = "Q", type = "BOOL" }, { name = "ET", type = "TIME" } } },
   TOF = { inputs = { { name = "IN", type = "BOOL" }, { name = "PT", type = "TIME" } }, outputs = { { name = "Q", type = "BOOL" }, { name = "ET", type = "TIME" } } },
@@ -121,14 +128,23 @@ M.PARAMETERS = {
   I_BCD = { inputs = { { name = "IN" } }, outputs = { { name = "OUT" } } },
 }
 
+--- Get parameter info for a built-in instruction
+--- @param name string The instruction name
+--- @return table|nil Parameter definition with inputs and outputs arrays
 function M.get_parameters(name)
   return M.PARAMETERS[name:upper()]
 end
 
+--- Check if name is a known function block
+--- @param name string The name to check
+--- @return boolean True if known function block
 function M.is_known_function_block(name)
   return M.FUNCTION_BLOCKS[name:upper()] or false
 end
 
+--- Check if name is a known function
+--- @param name string The name to check
+--- @return boolean True if known function
 function M.is_known_function(name)
   return M.FUNCTIONS[name:upper()] or false
 end
diff --git a/lua/scl/db_parser.lua b/lua/scl/db_parser.lua
index cab480c..bc1116c 100644
--- a/lua/scl/db_parser.lua
+++ b/lua/scl/db_parser.lua
@@ -3,10 +3,30 @@
 
 local M = {}
 
+-- Cache for parsed DBs: { [db_name] = db_definition }
 local db_cache = {}
 
+-- Inline DB definitions (from comments): { [db_name] = db_definition }
 local inline_dbs = {}
 
+-- DB Definition structure:
+-- {
+--   name = "DBName",
+--   title = "Data Block Title",
+--   version = "0.1",
+--   comment = "User comment",
+--   members = {
+--     {
+--       name = "memberName",
+--       type = "Bool" or '"OtherUdt"',
+--       attributes = "{ S7_SetPoint := 'True' }",
+--       comment = "Member comment",
+--       is_udt = false -- true if type is a quoted UDT reference
+--     }
+--   }
+-- }
+
+-- Register an inline DB definition (from comments or other sources)
 function M.register_inline_db(db)
   if db and db.name then
     inline_dbs[db.name] = db
diff --git a/lua/scl/multiline_params.lua b/lua/scl/multiline_params.lua
index 81e984a..7492093 100644
--- a/lua/scl/multiline_params.lua
+++ b/lua/scl/multiline_params.lua
@@ -1,5 +1,11 @@
+-- Multiline parameter filling for SCL function block calls
+-- Automatically expands FB calls to multi-line format with all parameters
+
 local M = {}
 
+--- Get interface for built-in FB types (TON, TOF, etc.)
+--- @param var_type string The FB type name
+--- @return table|nil Interface with inputs, outputs, inouts
 local function get_builtin_interface(var_type)
   local builtin = require("scl.builtin_instructions")
   local clean_type = var_type:gsub('"', ''):gsub("#", ""):upper()
@@ -14,6 +20,9 @@ local function get_builtin_interface(var_type)
   return nil
 end
 
+--- Get interface for custom FB types from workspace
+--- @param var_type string The FB type name
+--- @return table|nil Interface with inputs, outputs, inouts
 local function get_fb_interface(var_type)
   local fb_parser = require("scl.fb_parser")
   local clean_type = var_type:gsub('"', '')
@@ -26,6 +35,9 @@ local function get_fb_interface(var_type)
   return fb_parser.get_fb_interface(clean_type)
 end
 
+--- Check if a type is a known FB type
+--- @param var_type string The type name to check
+--- @return boolean True if type is an FB
 local function is_fb_type(var_type)
   local fb_parser = require("scl.fb_parser")
   local clean_type = var_type:gsub('"', '')
@@ -38,6 +50,9 @@ local function is_fb_type(var_type)
   return fb_parser.is_fb_type(clean_type)
 end
 
+--- Get list of known local variable names
+--- @param bufnr number Buffer number
+--- @return table Set of variable names
 local function get_known_variables(bufnr)
   local scl_vars = require("scl.variables")
   local variables = scl_vars.extract_variables(bufnr)
@@ -48,11 +63,17 @@ local function get_known_variables(bufnr)
   return known
 end
 
+--- Get the type of a variable by name
+--- @param var_name string Variable name
+--- @param bufnr number Buffer number
+--- @return string|nil Variable type, or nil if not found
 local function get_variable_type(var_name, bufnr)
   local scl_vars = require("scl.variables")
   return scl_vars.get_variable_type(var_name, bufnr)
 end
 
+--- Fill multiline parameters for FB call at cursor
+--- Expands single-line call to multi-line with all parameters
 function M.fill_multiline_params()
   local bufnr = vim.api.nvim_get_current_buf()
   local cursor = vim.api.nvim_win_get_cursor(0)
@@ -169,6 +190,11 @@ function M.fill_multiline_params()
   vim.api.nvim_win_set_cursor(0, { new_cursor_row, new_cursor_col })
 end
 
+--- Check if cursor is inside an FB call parameter list
+--- @param bufnr number Buffer number
+--- @param cursor_row number Current row (1-based)
+--- @param cursor_col number Current column (1-based)
+--- @return boolean True if inside FB call params
 local function is_inside_fb_call(bufnr, cursor_row, cursor_col)
   local function get_line_text(rownum)
     return vim.api.nvim_buf_get_lines(bufnr, rownum - 1, rownum, false)[1] or ""
@@ -214,6 +240,9 @@ local function is_inside_fb_call(bufnr, cursor_row, cursor_col)
   return false
 end
 
+--- Jump to next parameter in FB call
+--- Used for Tab navigation within parameter lists
+--- @return boolean True if jumped to next param, false otherwise
 function M.jump_to_next_param()
   local cursor = vim.api.nvim_win_get_cursor(0)
   local row = cursor[1]