"A column of type sql_variant may contain rows of different data types."
fwiw: I parse that sentence at least two ways. I believe the correct meaning is that the field value on any particular row may contain a different data type than some other row.
It might also be taken as: the sql_variant type is something similar to an array that could contain rows of data. I'm sure there are a few programmers out there who would like to store a complex object like an array in a single row.
My assumption is that most DBA here simply avoid this type. I'm curious to know how/why this type is being used in the real world - and whether it can be indexed.