If you want to later add gravestones to this same lot, you must first retrieve all the tombstones already there. Pick up the lot, put it in the lot bin and then place it back down.
I haven't found this to be necessary in my Neighborhood cemetery. I've moved seven graves to it (in three separate moves ~ five at once from the Gothier Greens lot Downtown [I lost one previously], plus two individual moves from homes in the Neighborhood), without first binning it, and they're behaving. <<knock wood>> I've never had to bin that lot since building it.
Maybe binning is only necessary for secondary neighborhoods (University campuses & Downtowns).
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