I worked for Deloitte once upon a time and I would personally slap someone for coming up with the title "Specialist Master". A, the pic above is spot on for what comes to mind with the title. B, cannot believe the PC police allow this. So if you are a Specialist, you work for a Specialist Master? If the junior is a minority, that seems perfectly great they work with someone called master. Hell, in homebuilding its now called an Owners Suite and not Master Bedroom just to avoid the potential of negative connotation.
All that being said, here is a quick link going over it:
https://www.quora.com/What-is-does-a-specialist-master-do-at-DeloitteBased on my knowledge / experience, it seems like specialist would have the focus on project delivery and being a complete SME without the pressure to sell. At the top level, this was delineated at some firms as difference between a partner / principal and a director. Director did not have the equity but also did not have as much demand on sales / growing the business / etc (though Director still needed to sell to an extent).
I would be wary of the specialist type role if you want to grow within the firm. At some point you either max out on the specialty or the opportunities for grand master dragon specialist of the highest order are few and far between.