The difference between the two is subtle and this is one of the shortcomings of the ACRONYM methods. Sometimes choosing the category or "rule" is harder than knowing which to use in the actual context.
Para is often translated as "in order to" in that context. Por is seen as the porqué, the reason or motive for.
In one of Paralee's lessons there is a visual aid that explains the difference in meanings. It shows something akin to a time/space line and shows that Por is used for beginning, at and through, while para is reserved for destinations and endings.
So if the context is a reason for or motive (beginning) Por is used and if it points to and ending target or destination like "in order to", then para would be used. Since one of the "rules" for para is "destination" (para ellos) this isn't hard to remember.
Just remember beginning motives, reasons por, results, endings, destinations para.
I'll see if I can find the visual aid. I found it more instructive than the ACRONYM "rules".