Services could be low-level services, like VPN thats realized with binary libraries, or custom scripts.
Most common services are:
When the service is used, node thats provide it issue an receipe thats should be signed by service's users. Every user should have key for receipe sign, key could be its wallet or provided from another wallet's owner. Wallet's owner issue special conditioned transaction with conditioned outputs, closed with the key that should be same with the key thats sign the receipe. Also services can issue orders, thats containes specific service request to the concrete participiant of the service or to indeterminate group of that participiants. Orders are propagated through GDB to another node in network.