What All Synced Photos mean?

"It is perfectly possible to add photos manually to the ASP special collection in LrC, those photos do not have to be contained in any collection (synced or not).."

Thanks @Jim Wilde 

So I gather from this that I could simply drag multiple files from folders in LrC, or even 'All Photographs' selected, directly to the "All Synced Photos" list and never need to create synced collections- correct?  (I would not be creating 'Albums' in Lightroom.)

Just some thoughts to ponder in this scenario, when you “import” images to Lightroom Classic the info with respect to image files is recorded in the Lightroom Catalog and so are any functions you carry out after the import. The Catalog file does not contain any actual image files or copies of the image files.

When you create a Collection in Lightroom this function not create a copy of the original image it is just data recorded in the Catalog file. You can add an original image to multiple Collections. Any changes/ edits you have made to the original image or if you have selected it in any of the Collections this will be applied to the original and the collections.

You can select specific Collections and have them synced to your Creative Cloud storage. All your other Lightroom apps Desktop  or Mobile apps will have access and any changes made with any app will sync accross all collections and back to the original image file. If you delete that original image or remove it from the Catalog it will be removed from all synced devices.

The Synced Collections will be added to the All Synced Photos collection as will any images you add to All Synced Photos and any Images synced from Lightroom Desktop and Lightroom Mobile.

If you select images in Lightroom Classic “All synced phots” and delete / remove them they will be removed from all synced devices including all collections in Lightroom Classic.

The original image files in Lightroom Classic will remain in the Catalog and in the storage folder / disk in your system, edits will remain intact.

All Synced Photographs contains all photos that are currently being synced between your LR Classic catalog and LR Cloud (but not synced videos, due to a longstanding LR bug).  A photo doesn't necessarily have to be in a synced collection to be synced between LR Classic and Cloud. 

If you remove a photo from All Synced Photographs, it will be removed from LR Cloud but stay in your LR Classic catalog.