X2 link is used to exchange information related to eNodeBs as well as for UE mobility. In LTE architecture, eNodeB has mandatory interface i.e. S1 between an eNodeB and MME. X2 interface is an optional interface between eNodeBs.
An eNodeB can initiate X2 link based on SON feature ie. ANR or manually as well. Operators prefer ANR since it is required minimal human intervention for managing and controlling the network. Most of the functions are automatic. Each cell maintains NR table for its neighbor cells. Each entry in NR table may consist of many attributes like No X2, No HO, No Remove etc.
Each cell performs actions towards neighbor cells based on the entry presents in NR Table.
Please x2-AP protocol specification 3GPP 36.423.