It can be one ITP covering all three parties or it can be two separate ITP's between client/contractor and contractor/subcontractor.
The most important part is the ITP must be agreed by all parties.
Generally the subcontractor will prepare an ITP that details all intended Inspection and Testing for the project.
This then goes to the contractor for review.
They can then add any contractor Hold/Witness points and if necessary make comments on things they may not be happy with.
This then goes to the client for review.
They can then add any client Hold/Witness points and if necessary make comments on things they may not be happy with.
When all three parties are happy with additions / deletions and amendments it is signed off and ready for use.
Hope that helps,
Cheers,
Shane