What do the abbreviations in the hire log mean?

The format of the abbreviations are Zx where Z is any of:

D - Distance counted by the distance counter (= running distance)

T - Time counted by the time counter (= running time)

C - Chargeable distance/time (whole units of distance/time charged - this doesn't include the current unit's fraction)

H - Overall distance/time while hired (from HIRED until the meter is FOR HIRE again)  (= hire distance/time)

and x is the unit of measure and is any of:

y - yards

m - meters

s - seconds

M - miles

K - kilometers

h - hours:minutes:seconds

x depends on whether the tariff is imperial/metric and whether you have enabled Menu > Settings > Interface settings > Format totals as hours, miles or kilometers.