What does the = in vim mean?

Cscope recommended mappings in the documentation:

Nmap <c-_>s:cs Find S <c-r>=expand ("<cword>") <CR><CR>

What is the meaning of <c-r>=. An assignment statement. <C-R> is the meaning of inserting a register value, = is the meaning of the expression register. So the sentence is to insert the value of expand ("<cword>") at the command line, which is the word under the current cursor.

In fact, after <C-R> and then enter <C-W> is the current cursor under the meaning of the word,<c-r><c-w> and <c-r>=expand ("<cword>") should be equivalent.

I will generally look for caller mappings to:

Nmap F:cs F C <C-R><C-W><CR>

The lookup symbols are:

Nmap S:cs F s<c-r><c-w><cr>

F The intention is to go to the point where the next letter appears, S is cc consent, delete the current line and enter the insertion mode. Neither of them is common to me.