A lot/most edi software indeed does have not only the 'parse' function but also the mapping. You do need both! A lot of the edi questions here are asked by 'edi starters', looking for a quick one-partner solution. If you use 'parse-only' edi software, you will need to build the mapping functionality yourself, re-building what is already available in all serious edi-software.
The guidelines of partners can indicate a different usage of the same transaction (like 204). Added elements is not allowed (but sometime they even do that:-( Some build a new mapping for every partner, some try to do this in one (complicated) mapping. I usually try to build one mapping, make it suitable for 'most' edi partners. But sometimes the usage is so different that it is better to build a specific mapping for that one partner. There is no magic formula to do what you want, I am afraid.