[NTG-context] 6 ways to ask for Times-Roman. Are they the same?

Hi all. Studying the pdftex.map file I found the following 6 TeX fonts that are used for Times-Roman: ptmr8y, ptmr8r, rptmr, rtxptmr, tir and tmrm108r. In the "no download" version of the map file, all these TeX fonts are mapped into Times-Roman. In the "download" the font utmr8a.pfb is embedded for them. Are they the same? Is there a preferrable one? What role does the encoding play here? Many thanks for your attention, --Stefan PS (Background): I try to write a type script for using the 14 base PDF fonts.