A few common considerations are listed below.
If you have an external drive that you want to back up, you can update your file selection to include this drive. Alternatively, if you want to back up the external drive on a different schedule than your local files, you can create backup sets to manage the file selections separately.
This default backup set often covers the majority of what should be included in your back up set. If you are concerned about the size of your backup set, you may choose to selectively delete folders or individual files from your backup selection. You can also choose to delete specified file types from this set, regardless of where they are stored.
The Code42 app and Time Machine complement each other well and they can be used on the same device, side-by-side, backing up your live data. However, for technical and practical reasons, you shouldn't try to integrate the two. Instead: