maybe it refers to the Gs interface, in the case of combined LA/RA update.
As far as I remember, the SGSN can receive a RA update from MS (through a packet connection) and forward it (as a LA/RA update) to the MSC/VLR.
I’m not satisfied by my explanation myself for several reasons, but that’s the only interface that is directly concerned by “location update”.
There is also the interface “visitor HLR – home HLR” : if the MS is roaming in a foreign network, the location update should be forwarded to the home HLR, isn’t it?
Well, that’s just ideas… I hope someone else can help 🙂