There are papers on this:
https://support.sas.com/resources/papers/proceedings/proceedings/sugi26/p009-26.pdf
Other people have asked the same question:
https://communities.sas.com/t5/SAS-Programming/What-is-the-difference-between-Missover-and-Truncover/td-p/31888
Plus plenty of other material on a quick Google search which should answer this question.
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