Where does this mysterious "Part 3" comes from?

w_s856

Context: https://cad.onshape.com/documents/7b6a91ec522a4dfa3bcd92af/w/16db1dd0c573e4f7ced45e8a/e/89ad5276f1ecdb27e889886e?renderMode=0&uiState=6672b4e7787cc04be3c55d66 (a cat feeder). Names of elements you can find in the drawing are italicized in the post below.

The Automated Cat Feeder (TM) I am building (and learning Oneshape as I go) is made of a few parts: a body, a helix and a bottom cap (more will obviously come, and the parts are incomplete). I neatly filed the various elements in folders but I see a Part 3 as part of my parts (I was expecting body, helix and bottom cap only).
Note to myself: I think that "cap" should rather be "lid" (English is not my first language)

This unknown Part 3 is a ring that I extruded as part of the "ring" section of the cap: these are two rings that I extruded at the same time, one to go on top of the body (it will be locked later), and one as a holder for the helix axis. I did not manage to make an all-in-one operation: the two rings and the actual surface of the lid. I added that element separately and merged it with the cap.

What I do not understand is how I ended up creating this Part 3. My understanding (please correct me if I am wrong) is that a new part is created when I "New" an extrusion, as opposed to "Add" it (in which case I must tell what to merge with). I "New" ring , and I "Add" bottom - which for me should make one single Part (← sorry for the awful English, I mixed the operations with the names to make sentences shorter)

eric_pesty

"Rings" creates two new parts (cap end and Part3) because the bodies are separate (you have to use "new" as you don't have anything to merge with yet).
"bottom" is set to "add" and merged with the cap end (as shown in the "merge scope" box at the bottom), you just need to add "part 3" in the merge scope to make the inner ring also part of it:

eric_pesty

If you select "part3" in the instance list, it will highlight the "Ring" feature that creates it.
The "bottom" feature adds to the outer ring but not the inner bit. If you want "bottom" to combine with "part3" you have to add "part3" to the merge scope.

w_s856

eric_pesty said:

If you select "part3" in the instance list, it will highlight the "Ring" feature that creates it.
The "bottom" feature adds to the outer ring but not the inner bit. If you want "bottom" to combine with "part3" you have to add "part3" to the merge scope.

Thank you for the answer. I am not sure what you mean by "add part 3 to the merge scope". Where can I do that?

eric_pesty

"Rings" creates two new parts (cap end and Part3) because the bodies are separate (you have to use "new" as you don't have anything to merge with yet).
"bottom" is set to "add" and merged with the cap end (as shown in the "merge scope" box at the bottom), you just need to add "part 3" in the merge scope to make the inner ring also part of it:

w_s856

Thank you, I understand now and it worked. 
Does it mean that when you create a single extrusion (in mode "New") from a single sketch that has two disconnected elements, there are two Parts created? Now that I wrote the question, it kinda makes sense I guess I was thinking that a Part is an object that does not necessarily be made of contiguous elements. Which in hindsight does not make much sense so I now understand everything