You can use the API explorer to find it.
Just look up the Issue object, and notice that one of the fields you have access to is Queue Topic ID. This basically means you have a way to refer to all the fields associated with the queue topic ID on the issue object.
Go to the references tab, click on Queue Topic to open it, and click on the link to go to the queue topic object.
Notice that one of the fields is the Parent Topic Group ID -- the topic group is the next level up from the queue topic and is a way to group queue topics together under one umbrella (I assume this is what you were talking about when you said "main queue topic"). Again -- an "ID" field basically means you have a way to refer to all the fields associated with the parent topic group ID on the queue topic.
So go to the references tab, click on the parent topic group field to open it and click on the link to go to the topic group information you need.
The textmode for your grouping basically ends up looking something like this:
group.0.valuefield=queueTopic:parentTopicGroup:name
group.0.valueformat=string
textmode=true
On your request report, it means "refer to the queue topic object on the request (which is possible since requests have the ID of the queue topic to use as a reference), then refer to the topic group object when you get to the queue topic in question (which is possible since queue topics have the parent topic group ID as a reference), and look for the name of the topic group once you get to the parent topic group in question".
