Without context that could mean a number of things, but the essentially the concept is that preindexing places items based on their properties such that they an index would always know where they can be found as a natural property of the data within, whilst post-indexing has items added to a system and stored in any location, with the index holding the information on both what indexable information they contain, and their area in memory. To use an analogy, preindexing is a bit like inputting paperwork and then adding each to the right part of the pile such that it is always in alphabetical order by name, while postindexing is like looking through an unsorted pile of work, and making an ordered list of the names along with where in the pile each is.

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