Good question.
Generally, CAP V3 supports Mobile Originating (MO) SMS from the VPLMN.
CAP V4 supports MT-SMS.
In the past I have used CAP V3 for only MO-SMS from a VPLMN. In the Home Network CS1+ (Ericsson) was used and the VPLMN traffic cases supported CAP V3. Combination of OICK and CSI were used.
OICK – Originating IN category Key (specific to Ericsson’s IN solution)
CSI – CAMEL Subscription Identity
VBR/ Wallis Dudhnath