! I am learning from this thread. I know a little about Rockler's T-track, because it is the closest woodworking store to me, and I have used their stuff.
I do not fully understand TerryQ's comment about the Rockler T-track. He might be referring to the height of the slot inside T-tracks, which can differ, or he might be referring to the thread sizes of the T-Slot bolts and accessories that Rockler sells. I will address both, because I have encountered them both.
As far as I know, all T-tracks are 3/4 inch wide with a 3/8 inch wide slot inside. T-track internal slot heights can vary, and this may be what TerryQ was referring to. I ran into it when a bolt or nut that fit in one T-track did not fit in another. That can affect the depth of the dado that you cut when you install the T-track, too.
Until this thread, I had not grasped the subtleties of T-track design, shapes, and sizes. I did a web search to learn more, and recommend this article that explains it far better than I can:
T-Slot Bolt, Knob, and Accessory Thread Sizes
TerryQ could be referring to the thread sizes of Rockler's T-slot bolts and the matching knobs and accessories that go with them? It is important to note that they all fit in the same T-track (probably any T-track). The T-bolts (which have an oval head) come in two thread sizes: 1/4 - 20 and 5/16 - 18. It doesn't matter much which one you use. Thinking about it now, maybe the 5/16 ones are considered stronger or "heavy duty." The essential point is that the bolt and the knob (or whatever) must match. Most of mine come from bags of mixed parts, so I have both the 1/4 and the 5/16 stuff, darn it, and Terry is right that it is annoying when you accidentally get a mismatched set. Keeping them sorted by thread size helps.
(Hint: If you go into the store, they will be grouped together by size: 1/4-20, 5/16-18, and a third group to avoid: "3/8-16".)
You don't need fancy T-bolts and knobs and stuff for everything. Depending on the jig or project, ordinary nuts and bolts and washers work fine, but be sure that they fit in your T-slots first. I can say that Rockler's T-bolts are easier to align and slide into T-slots compared with regular hex bolts, but that is an extravagant luxury, not a necessity. Just make sure your hex bolts and nuts fit your T-slots due to the internal height issue mentioned above.
In case it isn't obvious, if you plan to install nuts in the T-slots, then you must be sure the corresponding bolt is the correct length. Too short, and it won't reach or get enough threads into the nut. Too long, and it will dent or cause the nut to bend the T-track itself.