[SM] CRE tile table too large - what was overwritten?

[Pluribius]

I've been messing about with a major newbie's hack (it was fun to play despite the atrocious graphics so I figured I'd see if I could get Project Base type graphics edits and general cleanup working) and I noticed he never repointed his CRE tile table in spite of trying to use the white XX tiles. I tried altering an SCE and it gave me a warning that the CRE table was too large and might overwrite things (which, given the additions, indicates that newbie already overwrote things). What was overwritten, and was it important?
(I repointed the CRE table now, so I could change it back, but I'd still like to know what exactly was overwritten)

I checked someone's ROM/Bank map and it said something about BGi, but I don't know what BGi stands for.

[Mettyk25jigsaw]

BGI....That's Background data...