The standard M84 is intended for infantry use, and is derived from the Soviet-made PK machine gun. It differs from the PK-series in that it has a solid wooden stock and an unfluted barrel. It is also configured for tripod mounting (like the PKS) and comes with a scope mount (like the PKMSN).
The M86 variant is based on the PKT, and is designed to mount as a coaxial weapon on tanks and other combat vehicles. The stock, bipod, and iron sights are omitted from this version, and it includes a heavier barrel and electric trigger.