CRM Enable Sync is a system field that all the CRM integrations use to determine whether the Acoustic Campaign contact record must be synchronized to CRM. The value is set to either 0 or 1. If you set the value to 0, you do not want to sync to CRM. If you set the value to 1, you want to sync to CRM.
You can build a classic query against the 'CRM Enabled Sync' field. However, this field type is numeric. If you build a classic query using CRM Enable Sync equals Yes, you might get an exception error. Conversely, if you build a query using CRM Enable Sync equals 1, you will get the expected results.
For more details, please reference the "CRM" feature database field usage article.