Hi,
I've extensively searched the documentation but I can't find explanation about column tag. I am especially wondering what the available types are for sql-type attribute of column tag.

sql-type is just what it says. It isn't used in the mapping (I think) but just for DDL generation using hbm2ddl. It is not translated, so you can use any type supported by your DB, with the downside that you lose portability moving to a different DB. If you don't need to specify it, it is probably better to let NH infer the column type.

For some constructs <column> is required (e.g. <any> mappings, custom value types) but even then the sql-type can be inferred.