The slash-sheet is an appendage to the standard that specifies what a system of resin and fibre material has to comply to for it to be allowed to claim conformance to that slash-sheet. It is specified for both pre-preg and base material in that slash-sheet.
This is also why a system can be /24 /121 /124, it just means that the requirements given in all those sheets fit what the system offers.
For exact specifications you must always use the product name and manufacturer to identify the material, since many different systems by different manufacturers can conform to /24 or /41, and specifying /41 might mean you get one that is "better", but has different constants. Since you are selecting a material for HF and/or controlled impedance, you want to know the constants for sure, so you need to know which datasheet to look at.
You can use a slash-sheet specification to ask a Fab for all the material systems they offer that conform to that or those sheets and then compare the specifics of those. In that sense you can use a slash-sheet to pre-select a group of materials that will hold up in your intended environment or that are likely to be close to the constants you'd prefer.
For example /101 states a loss tangent, MAXIMUM at section 7 at 1 MHz of 0.035 for both <0.5mm and >0.5mm thickness. But that's not to say that a specific manufacturer can't make a /28 material with a maximum loss tangent of 0.03 for >0.5mm thickness and say it's also /101 compatible, because it is.
To note: I just picked two numbers and found the first difference I saw, it's possible /101 and /28 do have some other conflicting exclusionary properties that I didn't notice in 30 seconds, it only serves as an example.
Not to mention your manufacturer might specify another loss tangent at a much higher frequency that differs slightly, but their loss tangent adheres to the 1MHz specification. They can then be /101, but you'll be assuming the wrong number!
EDIT: For reference, my limited amount of numbers comes from the voting preview of IPC-4101-B edition, released 2006, because I find it nonsense to constantly pay big consortia of companies for their releases, even if it's "only $140" per release. The participating companies can easily foot the bill and should, to get more people to use those specs in their designs.