The units of PV.
This integer is a sum of some unit constants (CALCEPH_UNIT_???) and/or the constant CALCEPH_USE_NAIFID.
If the unit contains CALCEPH_USE_NAIFID, the NAIF identification numbering system is used for the target (NAIF identification numbers for the list).
If the unit does not contain CALCEPH_USE_NAIFID, the old number system is used for the target (see the list in the function f90calceph_compute()).