A position is an ordinal in the results Apple returns for a search, out of roughly 190 to 200 results in total, on the day we looked.
In the top 25 that ordering is stable enough day to day to be worth calling a rank. Below that it moves around for reasons that have nothing to do with the apps involved, so a position of 140 and a position of 160 are not meaningfully different and we do not present them as if they were.
An app that does not appear at all is outside those ~190 results — not absent from the App Store, and not necessarily far away. It is simply past where we can see.