A splined bore end yoke slides onto the splined output of a transmission, transfer case or differential and is retained by a nut. This is the common arrangement across most drivelines.
Two figures decide fitment: count the splines on your output shaft and measure across the spline tips. A 27-spline 1.188 shaft and a 31-spline 1.375 shaft are not interchangeable.
Splined bore yokes transmit torque through the spline flanks rather than through a key or a clamp, which spreads the load around the whole circumference. The yoke seats against a shoulder on the shaft and is held by an end nut, so the spline fit governs both concentricity and how the yoke will run once torqued down.
Series determines which u-joint the ears accept; spline count and spline diameter determine which shaft it goes onto. Both are published on every part page, and both have to match.