In article , Jonathan Morton
writes
Linking in with the other current thread on signal visibility, I presume
that class 1 is "8 seconds at line speed on the road", class 2 is less
strict than that (but still used in cases where they need to be visible from
a moving train), and class 3 is only for cases where the view will be from a
train standing at a platform waiting for the RA. Is that right?

GK/RT0031 says:

B27.5.1 Performance category 1 equipment
All equipment meeting the requirements of performance category 1 shall
be designed to be readable at speeds up to 125 mph.

B27.5.2 Performance category 2 equipment
With the exception of the banner repeating signal, equipment which meets
the requirements of performance category 2 shall be designed to be used
only where the approach speed of trains requiring to read the indication
does not exceed 60 mph.

B27.5.3 Performance category 3 equipment
With the exception of independent position light signals which are
pre-set by a main route, equipment which does not exceed the
requirements of performance category 3 shall be designed to be used only
where there is no requirement for the aspect / indication to be read at
a speed exceeding 15 mph. For this purpose, it is assumed that 15 mph
will not be exceeded by:
a) movements contained within the confines of a yard or siding
b) movements starting from rest, or virtually at a stand at a main stop
aspect
c) movements proceeding under the authority of a position light aspect.

B27.5.4 Performance category 4 equipment
Semaphore main signals shall be designed to be readable at speeds up to
100 mph.

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