The question is: At what point do you want to filter records?
If in a process, use Value Check, enter the attribute and check its value. Then records that pass the check go down the 'Pass' output filter and can be used in the remainder of the process. [If you have multiple values that you want to check against, use List Check rather than Value Check.]
If at snapshot time, use a WHERE clause on the snapshot. You can define simple filters like this one in the UI if you don't wish to write the SQL.
If at runtime (define the set of data to run when running a job), use an externalized option on the snapshot (if you want to vary the filter on different runs using a command line argument or using a run profile), or just define 'reader' filtering in the execution preferences of the job (if you always want to select the same data for the job, but want the whole of the data normally snapshot for other jobs).