The airliner being developed in Russia and branded as the Irkut MS-21 is a Boeing 737 and Airbus A320 competitor. Well, in theory anyway. The airliner is being developed to fit into the same capacity / range of those two product lines but it is a long haul and who knows if the airliner will ever be truly launched into production.
The Yak-42 was a three engine rough runway, t-tail design built in the hundreds by Russia during the Soviet Era. For a Soviet aircraft, it was a pretty good airplane. It resembled a 727 and had roughly the same capacity of a 717-200.
The actual progression was Yak-40 (a regional 3 engine airliner capable of rough fields and carrying about 40 people), then the Yak-42 and a Yak-46 and Yak-242 were conceived and even went through design exercises before being cancelled.
Now they want to change the name to the defunct Yak-242 and allege that the MS-21 is derived from that study. It’s a branding thing.
The problem is that the Yak has no positive brand image anywhere but in Russia. No one thinks of the Yak-42 and says “Yeah, they should built something like that again.” Calling the airliner Irkut MS-21 put distance between it and every other bad experience made in Russia.
But Russia clings to things and in this case it’s going to cling to the name Yak and believe that that name is going to win.
It won’t but that may be moot anyway as it remains to be seen whether or not Russia can build an airliner that interests the world. So far, the Sukhoi Superjet ain’t.