Layer 2 adjacency is when two or more devices need to be in the same VLAN, This leads to design complexity in order to extend these VLANs across multiple downstream switches.
Adjacency table - Nodes in the network are said to be adjacent if they can reach each other with a single hop across a link layer. In addition to the FIB, CEF uses adjacency tables to prepend Layer 2 addressing information. The adjacency table maintains Layer 2 next-hop addresses for all FIB entries.
To conclude, the process ID is local to the router, used internally to identify different OSPF processes that you might want to configure on a single router. Two OSPF routers can form adjacency over different OSPF Process ID's. You can run multiple OSPF processes on a router, using the Process ID as a distinguisher. You can have two backbone areas on the same router, or same areas on the ...
How is it possible to form an adjacency over different ospf process-id
Glean adjacency - When a router is connected directly to several hosts, the FIB table on the router maintains a prefix for the subnet rather than for the individual host prefixes. The subnet prefix points to a glean adjacency. When packets need to be forwarded to a specific host, the adjacency database is gleaned for the specific prefix.
By including the proposed missing figure, the terms in the original question begin to make sense. R1 will have a 2-WAY OSPF neighbor relationship with R2 without forming an adjacency (i.e., R1 and R2 are neighbors but did not become fully adjacent neighbors). This is what the original question was seeking.