I cannot find any rule for the definition of units in DATEX, neither in the methodology nor in the extension guide.
The assumption of using only base units of the International System of Units seems not to be valid, as there are things defined in DATEX Level A like 'Tonnes' (but not: 'Kilograms') and 'KilometresPerHour' as well as 'MetresPerSecond'.
So is there any rule for defining units?
(Matter of this issue is the parking model, where among others units like 'Hours', 'Minutes', 'Kilometres' are required).
The DATEX II Methodology policy is to recommend but not prescribe the use of SI units. A corresponding remark has been included in the Methodology document from v2.3 on and will be included Part 1 of the standard.