The best way to understand default macros is to examine their definition with the Macro Editor. For example, consider the macro titled "Begin/End." Suppose you wish to use an environment like the theorem environment. Type the word "theorem" and select it. Then choose the "Begin/End" macro. The word "theorem" will be replaced with the text
with the cursor placed on the line between this pair.
Now examine the begin/end macro code:
Text in the macro will be inserted into the source file. Each occurrence of the string #SEL# will be replaced by the text selected when the macro was invoked. If no text was selected, #SEL# will be replaced with an empty string. The cursor will be placed at the end of the inserted text unless the text contains the string #INS#, in which case the cursor will be placed at that location.
Using this knowledge, it is easy to understand and modify the default macros.