Underscore in Asterisk dialplan template, and without?

Question

Asterisk

The underscore is the beginning of the pattern. Could it be, for example, not exten => _3XXX, but exten => 3XXX?

Answer 1

Underlining is a sign of a pattern.
exten => 3XXX- string '3XXX'.
exten => _3XXX- pattern '3[0-9][0-9][0-9]'.