question about compilation-error-regexp-alist

[wuxch2000]

the compile.el's comment says

"Each elt has the form (REGEXP FILE [LINE COLUMN TYPE HYPERLINK
HIGHLIGHT...]).  If REGEXP matches, the FILE'th subexpression
gives the file name, and the LINE'th subexpression gives the line
number.  The COLUMN'th subexpression gives the column number on that
line."

it's easy to understand how compile.el know FILE and LINE content, but
how can compile.el know witch TYPE the message is?