I do apologize as my question was not very clear.
I know what a "metadata" is, the question should have been "Why would the metadata (or catalog) definition be different from the physical implementation?"
From what I understood from your original post, if you change the field length of an existing table from INT to SMALLINT, and assuming existing data is SMALLINT compatible, then your metadata definition changes (and will show SMALLINT when you query the catalog), but the field can still hold INT values for future rows inserted.
What I would expect is after the field length change I cannot insert values greater than SMALLINT's.
I did not see the point of the metadata being different from the physical implementation, I have worked with UDB, Teradata and Oracle and I have never seen the Catalog being different from the physical implementation.
Did I not understand the original post correctly?