t-sql 2008 r2 parameter values

In t-sql 2008 r2 I would like to know how to set a variable value that would be similar to

declare @avar varchar(3)

set @avar in ('ABS', YUR', 'UNV', 'MED')

The @avar character can be one or all of the values listed above.

I want to be able to set a variable like this since the user can select one, 2, 3, or 4 of the values listed above in an ssrs 2008 r2 report.