How to define custom sync set criteria

The queries in the drop down are the queries that have been created inside of Act!. If you want to create a new sync query, Login into the Act! Database and create and save a new query. (You can create and save a new query in the Query folder by using the advanced lookup in Act!.) 
After, you have created the new query you will need to refresh the ACP cache. (For instruction on how to do this see the Support links section, Diag Reports.) Once the cache is refreshed, the new query will be in the drop down.  Select the desired query.
Finally, on this page under the All Users column, there are check boxes for each sync set. If this box is checked, it means that ALL users, both now and in the future, will be able to sync with this sync set.