A command is an alias that does not return a value that is called by using its name preceeded by a / (or whatever you have set your command character to) and followed by a space and then any parameters you wish to pass into the alias. This is equivalent to a Turbo Pascal Procedure or a Visual Basic Sub. For example:
/MyCommand parameter1 parameter2
An identifier is an alias that does return a value (even if that value is $null) that is called by using its name preceeded by a $ and followed immediately by your parameter list (if any) surrounded by parentheses and directly connected (no spaces between the alias name. This is equivalent to a Turbo Pascal or Visual Basic Function. For example,
var %variable = $MyIdentifier(parameter1, parameter2)
Nesting can be done in many ways using different control structures, such as If-ElseIf-Else, While or Goto. Goto's are not generally considered to be nested, nor would any programmer worth his pay remotely suggest that you use them that way. THAT is what leads to "spaghetti programming," which is difficult to comprehend and is a nightmare to debug/maintain. You might have If statements nested inside other If statements; you might also have nested loops using While.
Another form of nesting is called recursion, where a command or identifier calls itself. mIRC does not directly support recursive routines, though you can fake them with timers as long as you maintain your own "stack" so you unwind it in the correct order and "return control" whence it originated. That makes it "possible," though certainly not a beginner's exercise.
