CUNITn (floating) is the units in which both CRVALn and CDELTn for axis n are expressed.
Units identified using this keyword should be chosen using the principles defined in section 18.104.22.168 and must conform to all the requirements in that section.
An axis may be described by more than one label. For example, a spectrum may be described by frequency, wavelength, and velocity, only one of which can be linear. GC propose reserving the following additional keywords:
The values of m may run from 2-9; the original CTYPEn, CRPIXn, CRVALn, and CDELTn keywords are used for m=1. Note that the reference pixel need not be the same for all descriptions of an axis. The values of a given pixel in the different units must be consistent, e.g., 1420.4 MHz and 21.1 cm. There are no defaults assumed for ; all of these keywords should be specified.