questions for "primary=true" and mmsc

["allstars.chh" <allstars chh gmail com>]

Hi, 
I am trying to use this service provider on mobile phones

But I got two questions when I see the serviceprovider.xml

1. 
In the provider tag, there's an attribute 'primary=true', 
I got an example here [1], 
In that case, one of T-Mobile's mcc-mnc has the same code with a provider called "Ben".(both mcc is 204, mnc is 16). 
So the attribute 'primary=true' is added to indicate that we should always select T-mobile when mcc & mnc got collisions. 
My question is, in this case, under what circumstance will the MVNO 'Ben' should really be selected, instead of the primary one(T-mobile)? 


2.
I found some apn whose type is mms, but where can I find mmsc??

thanks


[1]: https://bugzilla.gnome.org/show_bug.cgi?id=671372