On the other hand HST▸ includes loads of DMUs▸ that are not actually HSTs, including such as Voyagers.
The names given to the differential linespeeds aren't actually particularly meaningful. According to one of the sectional appendix documents I've downloaded from the Network Rail website; as well as IC125s with at least 3 coaches being permitted to run at
MU▸ linespeeds, class 185s and 390s aren't allowed to run at MU speeds and Sprinters aren't the only units allowed to run at SP (Sprinter) linespeeds, classes 165, 166, 168, 170, 171 and 172 are also permitted.
As you say, some trains which are not HSTs are permitted to use HST differentials, namely: 158s, 159s, 168s, 170s, 171s, 172s and 175s. Although most pepole on here seem to use 'HST' to mean trains formed of mark 3 coaches with a class 43 at each end, Voyagers (and class 180s) are technically HSTs
the definition being that "Rolling stock must have a minimum[clarification needed] speed of at least 200 km/h (124 mph) to be considered high speed". That's why I try to remember to refer to 'trains formed of two class 43s sandwiching mark 3 coaches' as IC125s (
INTERCITY 125s), as that unambiguiously refers to that type of train. The IC225s (
INTERCITY 225s) on the east coast main line are capable of at least 125mph (the locos at least are still 140mph capable according to their data panels, I keep forgetting to check the coaches) are also HSTs, as are the class 373s on HighSpeed 1 of course.
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