Not exactly, X is for partial express route... smethin like our
Ffw but at normal prices... normali for routes to far out places
like Yuen Loong... 3 digit buses beginning with 1XX is for the
normal harbour crossing, 6XX for eastern harbour crossing, 9XX for
western harbour crossing...
P is for PEAK period operations, S is for special
despatch[normali 1-5 buses]
Besides 1xx routes, there are also 3xx routes. This route
numbering also utilises the cross-harbour tunnels, but only plies
during peak hours, with the exception of 307,
which is a full day service.
To further add on to TIB1234T's queries about suffixes,
M is mainly for routes plying to MTR stations (HK's MTR = SG's
MRT). Do note that there's also a 'K', which is for KCR plying
routes (still used even after merger).
R stands for residential routes but can mean something else as
well. Have limited operating hours.