it depends on the contents to be transmitted in the paging. you know that one paging request can contain more than one paging, based either on IMSI or on TMSI . Each type corresponds to one possible mix between IMSI and TMSI pagings in the same Paging Request. 1 to 4 paging commands can be merged into 1 paging request :
Type 1 :
1 or 2 IMSI
1 or 2 TMSI
1 IMSI + 2 TMSI
What chooses to send an IMSI or a TMSI paging… that is certainly a MSC choice. Any idea ?