BSP threads come in 2 basic types
(1) BSPP [last P is for parallel] BSPP also used to be designated BSPF [still the same parallel thread with the F meaning "fitting"] i.e. it was used for plain [not tapered] pipe fittings. The current correct terminology for BS parallel threads is the letter "G" written either G BSP or BSPG.
(2) The second BS pipe thread type is tapered and its terminology is far less confusing! Usually written just BSPT. Helicoil type inserts are not recommended for taper use and the dimensions in the table below are for parallel threads.
In BSP threads it is quite common for a tapered external thread to be used with a parallel internal thread and this generally gives a sufficient interference seal unless for very high pressures.