The voltage specified is the AC output from the transformer. If you want more information search for "RMS voltage". It is heavy reading if you're not schooled in mathematics and electronics. It's too heavy for me. I'm not well schooled in either mathematics or electronics (only finished secondary level education in those subjects). All I know is that the DC voltage is usually the AC voltage multiplied by the square root of 2.

25VAC after full wave rectification is 25 * 1.414 = 35VDC
50VAC after full wave recitifcation is 50 * 1.414 = 70VDC

The AC voltage is nominal. It will depend on your actual line voltage and the transformer winding that you select. Measure the AC input and do the arithmetic. It should closely match.