It is also called “castellated holes”. These are plated holes cut 
through on the board edge and used to join two PCBs either by direct 
soldering or via a connector.  During the production, it is necessary to 
pre-process the semi-hole position before etching circuit to make sure 
the semi-hole position is smooth and no burr.

Process capability:

The hole diameter on the edge is minimum 0.6MM, and the half hole should 
be drilled inside the boards ≥0.3MM

Notice:

The spacing between castellated holes should be greater than 0.55MM, or 
it will easily cause short circuit.

For other PCB technology, such as Countersink, Depth Routing, please 
visit the below website:
https://www.kingcredie.com/Ab_index_gci_35.html