What would the equivalent of a label be in RTC?

Not knowing what a label is in your context it's difficult to answer with certainty, but I'm guessing you're talking about labeling a group of source files with a meaningful name?

If so, then the equivalent would be either a baseline or a snapshot (which is a set of baselines).

If it is a locked label, then it would be an RTC baseline or snapshot, as Jeff indicated.
If it is an unlocked label, then it would be an RTC stream or baseline.

One more suggestion in addition to the comment by Jeff and Geoffrey, I'm assuming "label" means a meta-data attached to versions under source control.

RTC source control has a mechanism called "user properties". This can be shown via "Select File" -> Properties (ALT-Enter) .



Unfortunately, "user properties"  can not be searched by SCM search. I created an sample code to display user properties. This can be obtained from JazzHub - "Jazz In Flight" project.
 
 https://hub.jazz.net/project/rschoon/Jazz%20In%20Flight

Hope this helps !