Thanks for Yutaka’s reply.
I means:
**Monitor Clipboard command**
Use this command to monitor changes of clipboard and insert the changed content into the end of current document(Event if EmEditor is not current window). This command can be used for abbreviation of cut-and-paste operation. Any text which is copied or cut to the clipboard will be inserted into the current document. A check mark appears next to the menu item when this option is active(or button on toolbar is press-down).

When EmEditor is not current window,can macros or plug-ins monitor changes of clipboard?

EditPlus have this function,you can feel it.
EditPlus/Document menu/Monitor Clipboard
Monitors and insert the changed contents of clipboard.