The NAV mode is used to intercept and follow a VOR radial (navigate towards or from a VOR station). This can be also called VOR mode. It can also be used to make a LOC (localizer) approach, when the GS (glide slope) signal has failed or is simply missing. BTW, you can practice this with our module, failing the GS signal in the ME failures section.
The VOR APR mode is used to perform a VOR approach. A VOR approach is a type of non-precision approach that can be used in IFR conditions at certain airports when, for example, there is no ILS available. The mode operates in an identical way to the VOR mode (NAV mode) but with optimized gain, which provides more precision for the VOR approach.
The BC (Back Course) mode is used when making a BC approach. A BC approach is a type of non-precision approach that uses the back beam of the ILS localizer signal. For example, when there is no ILS for one of the runway directions you can use this type of approach, since all ILS localizer antenna array radiate not only the frontal beam, but a back beam also, of course with lower intensity and lower precision probably. Obviously, to make such a type of approach the corresponding IAL chart has to be published by the country’s civil aviation authority.
APR mode is used to make an ILS approach, that is, full ILS with both LOC and GS signals operative. If the GS signal is inexistent or inoperative, you can make a LOC approach and use then the NAV mode of the FD, as described in first paragraph.
See the attached pdf for further explanation. Explanation of some of the C-101CC FD modes.pdf