Need help what we can use in solar query

It might be easier to try and understand what you want to do

but which variable we can use with in solar query

You can use solar syntax on basically any field stored in the index, which we don't document them all.  If I knew what you wanted to do I could help better

#set($query = "(type:forum && group:${fGroup} && section:${fSection}" )
#set($results = $core_v2_searchResult.List("%{ Query = $query")

my mean to ask is that if you will look into Code some one has used type,group,section as Query variable

then what are the all variable list that we can use into Query other then the type,group,section

For more clearance you can consider my requirement,

1. need to fetch ForumThreadList for multiple forum in one go

2. need to filter the ForumThreadList data or behalf of user role.

The following is a sample to filter by a specific forum and return threads.  No need to filter by group and section.

#set($query = "%{Query=$searchText,PageSize=10}")
$query.Add('Filters', "type::forum && (section::24 || section::1)" )

#set($results = $core_v2_searchResult.List($query))

thanks Brian Dooley will you please help me on 

what are the all variable list that we can use into Query other then the type,group,section

type::forum && (section::24 || section::1)

how can we use same approach by forum api($core_v2_forumThread)

for multiple forum

This is an older doc that may have the information you're looking for:
https://community.telligent.com/community/archive/w/7-api-documentation/31418/search-fields-by-type

There is not a way to list threads from multiple forums using the $core_v2_forumThread api unless you you want to pull all threads in a group or a group and subgroups.

There is not a way to list threads from multiple forums using the $core_v2_forumThread api unless you you want to pull all threads in a group or a group and subgroups.