The access point, usually provided by the ledger vendor, is the service gateway for a given business and so hosts the end-point to which business documents are sent. The gateway is an avaialbility mediator (allowing specific businesses to have intermittent connectivity) and is responsible for signature validation and submitting transactions to the notary service.
Specification: http://ausdigital.org/specs/ausdigital-tap/2.0/
Community discussion/support: ausdigital.org
Open source reference implementation: https://github.com/test-point/testpoint-tap/
Test suite: https://github.com/test-point/testpoint-tap
Live test endpoint: https://tap.testpoint.io/