"Phasing" in a driveshaft refers to the alignment between the Cardan joints on each end. Recall that the joint ends speed up and slow down as they go around (with the shaft in anything other than a straight line end-to-end,) and that the speed differences need to cancel each other out. This is why a double Cardan shaft needs a static alignment different from a single Cardan.

Phasing needs to see welds cut and rewelded to be changed, since you have to rotate the Cardan yokes at either end.

For typical R/I stuff, it's a good idea to mark a cap and yoke arm so that they can be lined up on reassembly, but it's not critical (and you can usually get away without doing it. I've been doing it for thirty years out of habit, and I do the same thing with clutches and flexplates - same reason. It's not necessary, but it's cheap insurance.) If the spline alignment were critical, then the splines would have an "alignment section" - just like on a Pitman arm (where three splines are joined into one. The Pitman arm will fit on the Pitman shaft only one way.) I've never noted a need to get splining joins spot-on at the end, and get them matchmarked in the middle primarily out of habit, again (a good shop will balance the two halves of a splined shaft separately, then verify the balance with the shaft assembled. I just don't always assume I'm dealing with a "good shop." Besides, misaligning central splines will screw with driveshaft phasing, as defined above.)