An Alias Type is a datatype that is defined in terms of a built-in datatype, but has a different name to convey more meaning. When you generate a Column that uses an Alias Type, the built-in datatype is substituted for the Alias Type. Alias Types allow you to define your Model's datatypes in more meaningful terms. You can use them to organize, re-use, and easily change the datatypes that your Columns use.
Alias Types are available for all databases. The Type dialog varies by database.