Pipe maintains the ID, tubing maintains the OD.
Most residential water line material is schedule 40.
Polyethylene, steel, PVC water line is usually pipe and thereby sized by the ID and the OD floats based on the pressure rating which increases or decreases the wall thickness and OD. I.E. 3/4" has an ID of 3/4", etc..
Copper, most PEX, CPVC is tubing and it maintains the OD and the ID floats based on the pressure rating which increases or decreases the wall thickness and ID. I.E. 3/4" has an ID smaller than 3/4" and an OD of 7/8", 1/2" is 5/8" OD and ID is smaller than 1/2" etc..