The API Specification for the add status service states that V7-V12 are number fields like the energy and power fields, as opposed to things like temperature, which are declared as decimal.
I could not find a definition of what number and decimal mean, but by looking at the examples given, number indicates an integer value, and decimal accepts floats.
However, when defining a tariff, you can select an extended field as a source for the tariff. Tariff rates are decimal (e.g. 32.3719 cents).
It seems when supplying decimal values for the V7-V12 fields in a call to addstatus.jsp that they are accepted, so could the API Specification be updated to reflect the actual datatype?