The %mkrel macro is used to combine the release number with the distro suffix.
The distro suffix is generated by other macros and for the final RPMS/SRPMS is pclos2012
The release number is incremented for each rebuild of the package with the SAME SOURCE version. So the first build of firefox 16.0.2 will be done with a %mkrel 1 which will produce:
If a change is made to the SPEC file or it has to be rebuilt for some reason then it will be changed to %mkrel 2 which would produce:
The increment of the release number ensures that Synaptic sees it as an update even though it is the same version (16.0.2) of firefox.
MDV was the suffix used by Mandriva and should be ignored.
Each packager has their own distro suffix so that when they submit their SRPMs to Neal it is clear who is submitting the package. For example my suffix is _terryn_
so when I submit SRPMS to Neal they are named like this:
Hope that helps,