Modular plugs go on the end of telephone and network cabling. The term "6P4C" specifies that these plugs are 6 positions wide, but only have 4 positions loaded. (The two outer positions do not have the metal contacts.) This plug will go into any 6 position wide jack.
A 6 position plug is physically what you are used to seeing on a telephone cord. Telephones (single line ones, anyway) only use the center two conductors. A two line phone would use the center 4 conductors.
These plugs are designed to work best with round cable (i.e. the old 4 conductor red, yellow, green, black phone cable) or with any category rated 8 conductor cable (you just don't connect all the conductors). They will work acceptably well with flat telephone cable. (The retention strength won't be up to spec.)
Use these connectors for terminating telephone patch cables, making phone cords, etc.