Phase Adjacencies are used to prevent underground mining and underground dumping. For example, you can mine Stage 1 and Stage 2 simultaneously, but Stage 2 cannot under-cut Stage 1. Similarly, you can backfill a pit while it is being mined, but you can't dump underneath an unmined block.
Phase Adacencies only set vertical dependencies between blocks. To finish an entire stage before another, see Custom Dependencies.
To add a Mining Phase Adjacency:
Select a Phase which is dependent on another from the Name column - i.e., Reserves/M1/P1/102
Using the Adjacent Phases dropdown list, select the Phase which it is dependent on - i.e., Reserves/M1/P1/101
To add a Dumping Phase Adjacency:
Select a Dump which is dependent on a Phase from the Name column - i.e., Dumps/Dumps1
Using the Adjacent Phases dropdown list, select the Phase which it is dependent on - i.e., Reserves/M1/P1
Set a standoff distance of 50 metres in the Dump→Pit Standoff column.
Setup > Phase Adjacencies.