There is only one syntax to specify your own multipliers but because of the different parameter options available, you get three different functional options.
If you want to match a given character a concrete number of times, you can simply specify the number of allowed repetitions inside curly braces. This doesn't make your patterns more flexible, but it will make them shorter to read. For example, if we were implementing a phone number we could type /ddd-dddd/. This is, however, a bit long and instead, we can just use custom multipliers and type /d{3}-d{4}/, which really shorten it up making it more readable.
Next, if you just want to set a minimum number of times that the pattern can appear, but don't really care about the actual length, you can just add a comma after the number. For example, let's say we want to create a pattern to make sure a user's password is at least six characters long; in such a situation, you may not want to enforce a maximum character limit, and can, therefore, type something similar to /.{6,}/:
The third variation on our custom multipliers is when you want to set a complete set of options, matching both, the minimum and maximum number of occurrences. You can do this by simply adding another number after the comma. For example, if we had some sort of comment system and we wanted to constrain the comments to be anywhere between 15 to 140 characters, we could create a Regex string to match this setup, for example, /.{15,140}/.
Now, I am not saying that the two previously mentioned examples are the best uses for this kind of Regex, because obviously, there is a much easier way to check text lengths. However, in the context of a larger pattern, this can be pretty useful.