Altchains adopting Bitcoin security through VeriBlock will use the VeriBlock Integration Point to record and maintain SPV-level knowledge of VeriBlock (and by extension, Bitcoin). VTB payloads contained in PoP transactions on the altchain are provided to the VeriBlock integration point, from which it constructs the SPV view. During fork resolution over a keystone boundary, the altchain will consult the integration point to determine the timeliness of publications of its keystone periods to the VeriBlock blockchain.
During normal operation, the integration point maintains a deterministic SPV-level view of VeriBlock based entirely on the VTB publications it has been provided. It maintains an association of these VTB payloads with their enclosing altchain block (by block number), so that altchain reorganizations can undo the state changes the unapplied blocks made to the integration point’s view of VeriBlock.
During fork resolution, the integration point is temporarily updated with VTB information contained within the challenging fork to ensure the integration point’s view includes information stored in both forks. After fork resolution, the altchain sends a command to the integration point to clear the temporarily-added VTB payloads, which returns the integration point’s view to one which only reflects VTB payloads in the main chain. In the event that the challenging fork is selected, the altchain still sends the clear command, and then un-applies VTB payloads in the previously-main chain back to the forking point, and applies the new VTB payloads from the new fork.
Appendix B of the document Proof-of-Proof and VeriBlock Blockchain Protocol Consensus Algorithm and Economic Incentivization contains details about the library's API and expected uses.