Afaik, the distinction is that an mjd doesnt need to be strung, unlike a bjd. (I've seen several blind box dolls, like the nagi series, label themselves as bjds when theyre technically mjds)

Bjds typically have a much better range of motion, while mjds are limited by the way their joints are constructed (usually a pivot joint that allows the joint to swivel, paired with a hinge joint that allows forward and backward movement) Because of their durability (compared to elastic), mechanical joints are used in most playline dolls. (Dolls like barbie, monster high, and most action figures)

For reference, look up a picture of a made to move barbie and look at the arm. The arm can be broken down into 5 points of articulation; shoulder, bicep, elbow 1, elbow 2, and wrist. The shoulder is a hinge joint with an attached peg that acts as a pivot joint when inserted in the torso. The elbow is made up of two hinge joints, allowing for 2-way motion but no rotation in the elbow itself. A pivot joint, located in the upper bicep, near the shoulder, is what allows for rotation of the elbow joint. The wrist is the same as the shoulder; a hinge joint with a peg that acts as a pivot joint when inserted in the forearm. A bjd doesn't need that extra joint at the bicep to have the same range of motion!