About FESS "Search Fields"

Fess: 14.11.0
Opensearch: 2.11.0

Is there any information about what
searching fields I can use in FESS search phrase.
For example, “allinurl” field, I’m trying to
understand how to use it. However, few useful information
I can find.
Could you teach me how to use it?
Besides, others available search fields I can use in Fess/Opensearch.

Thanks in advance.

Fess provides several features that are compatible with the Google Search Appliance (GSA).

Thanks your reply.
Frankly, I do not know GSA.
So do you suggest to me that I
should look for usage of GSA, in
which I can find out useful information
about FESS search fields, such as “allinurl”, “url”, “timestamp” etc.,.

There is no public information available on this topic. For further details, please refer to the source code or contact commercial support.

I see, thank you so much.