Stitch only shows indexed fields in the UI for MongoDB connections. The reason for this is that if you want to sync Mongo data and are going to add query parameters - which is what Incremental Replication does - undue stress could be put on your Mongo database. By indexing the fields you want to sync, that stress can be relieved.
For example: if you want to use the
updated_at field on your
customers table as your bookmark for Incremental Replication, all you'd need to do is index this field in your MongoDB. This field would then display in Stitch as available for syncing.