List of LRT "Key" Options

Is there a list of key options when using LRTs?  We have used FROM:

 

lrt:NAME;key=$FROM

 

And I see reference to RN in the config manual, but no where do I see what my other options besides just those too.  Can I key on multiple options?  For instance, I would like to key on the TO and the request-uri, and change both in the LRT:

 

route>
    <user type="E164">8005551234</user>
    <next type="regex">!(^.*$)!sip:3015551234@4.2.2.2!</next>
</route>

 

I'd like to match the 800 in the "to" and the "uri" and change it to the local digit number.

 

Thanks