DTM Data Generator Online Documentation - DTM Data Generator, From table fill method

By adding a WHERE clause, you can limit the list of values used from the specified table only to those that you need. The "Preview" page allows you to view top 50 values from the linked table (or less if there are no so many unique values). These values will be used in the generation process.

So, there are settings for this fill method:

"Use list items as a mask" means each list value will be processed as a mask before using.
"Use values sequentially" option means the program will use linked table values from the first to the last. When the list ends the program starts from the first item again.
WHERE clause helps you to use only required values from the linked table.
ORDER BY clause helps you to define order of values usage. Values order is important for "Use values sequentially" mode only.
Format defines the view of value. This option is applicable for numeric values only.