what is the difference?

If i take a batch of files, first put replaygain tags and then apply replaygain, is it any different from using the volume normalisation dsp?

Also, like replaygain has the 'album gain' option, why is there nothing similar for volume normalisation? Is volume normalisation same as applying replaygain (track gain)?

I am only interested in normalisation or applying replaygain, since my player does not read replay gain tags. So which one?

I understand the original files are altered, but is there some sort of compression involved too in both of these two processes?