Usually most poeple apply to both a transitional year and a PGY2 year in the same match. So for example, with radiology you would apply for both a PGY1 transitional, starting this year(or whatever year you are applying) and PGY2 spot in radio starting the following year in July. I am not sure if thats what you were asking?
However, say you only applied to a transitional year(or didnt match into your PGY2 position of choice) then at the end of your transitional year you could apply, either through the match or outside the match, for a PGY2 spot. You wouldnt want to apply for a PGY1 again unless you just decide you want to change into another field (Ex: transitional year then you decide to pursue OB/GYN).