The attribute MOOD is one of several that record so-called “variable properties”. You’ll find a helpful overview of these here:
The possible values for each of these attributes are documented in the file “semi.vpm” within the English Resource Grammar source, which you can obtain (for the most recent stable version) via this Linux command:
svn co - Revision 29259: /erg/tags/2020
But I am surprised that you report seeing “imp” as one of the possible values of MOOD. The only two intended values are “indicative” and “subjunctive”. The ERG distinguishes imperative sentences from propositions or questions via the attribute SF (for “sentence force”), with possible values “prop”, “ques” and “comm” (for command). If you really do see “MOOD imp”, please report the sentence you parsed with ACE that produced this value.