How do I format a cell for covariation?
What do you believe 'covariation' means?
English contains the nouns 'variation', 'variance' and 'covariance', but not
'covariation'.
If you mean covariance, it's still unclear what you want because covariance is
either a single numeric value, so the numeric format should be sufficient to
show significant digits (scientific formats like 0.000E+3 would be safest), or
covariance when there are several variables involved would be a symmetric
matrix, in which case you can't really cram the entire result into a single
cell, but should use a multiple cell range.