Standing agreements between ATC sectors can be found in the Flight Level Allocation Scheme (FLAS) document.
This includes all Letters of Agreement (LoA) with neighbouring countries as well as internal agreements between sectors within Romania (LRBB FIR).
Make sure you are familiar with this document before connecting on an APP or CTR position. You don’t need to learn everything by heart, but do need know where to look in order to find the required information rapidly!