The pitch diameter of a screw thread is the :

Electrical goods

B.S.F screw threads are generally used for Electrical goods.

Square

Square threads are used for transmitting motion or power. Eg. screw jack, vice handles, cross-slide and compound slide, activating screwed shafts.

Thread rolling

Screw threads are produced without metal removing by a process called thread rolling.

'V' threads

"v" threads are used for general fastening purposes.