[Xx] in this guide denotes the letters that will display if you have a GENOTYPE purchased. Genes which do not show this will not display in the genotype, thus the letters don't matter.
Whenever Xx is used in a general term, X refers to a capital letter and x refers to a lowercase letter. For example, if Xx as a general term could refer to Dd, XX to TT, or xx to cc. This is only used when talking about genetic probabilities and things that could refer to ANY gene a horse has.