Explanation of vba code in a word document having .docm extension - vba

Question

I have a Microsoft Word document with .docm format. A first glance it does not contain any macros (as when clicking the following on the ribbon; View -> Macros -> View macros pops up a window having an empty list).
But when enabling the Developer ribbon tab, and clicking the Visual Basic icon there, and then selecting the Document and ContentControlonEnter from the dropdowns in the VB window the following code appears:
Private Sub Document_ContentControlOnEnter(ByVal ContentControl As ContentControl)
Dim i As Long, j As Long
With ActiveDocument
If ContentControl.Title = "Classification" Then
ContentControl.DropdownListEntries.Clear
For i = 1 To .ContentControls.Count
If Left(.ContentControls(i).Title, 5) = "Level" Then
j = j + 1
ContentControl.DropdownListEntries.Add Text:=j & " - " & .ContentControls(i).Range.Text
End If
Next
End If
End With
End Sub
Selecting the other options in the dropdowns give only "blank" code (that is they contain only function declarations followed by theEnd keyword).
My question is what is the code meant to do?
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Details:
The Word document in question contains hyperlinks to parts of the same document and a couple of links to Word files and Excel files of the same folder. It also contains lots of content control boxes, which I'm guessing is the focus of the code (as the code contains the ContentControl keyword)

Answer 1

Content controls can trigger macros when the user enters and exits them. Microsoft made the design decision that all content controls should trigger the same "events" - Document_ContentControlOnEnter / Document_ContentControlOnExit - and that the code in the event needs to check which content control was entered / exited.
Content controls are considered as part of the Document because the Document can trigger events. That's why they're in (and MUST be in) the ThisDocument class module.
(Note: View Macros can only show you PUBLIC SUB procedures with no arguments that are located in "normal" code modules. Any Private Sub, any Function, anything that takes a parameter and anything in a class module will not appear in that list. So you can't use that list to determine whether a document contains any code.)
The If ContentControl.Title = "Classification" Then checks which content control was entered. (Note: it usually makes more sense to use Select Case rather than If, especially when the event needs to distinguis between multiple content controls.) What's inside the If only executes if it was a content control with the Title "Classification". (Note that more than one content control can have the same Title, so more than one content control could run the code.)
If another content control is entered, the event is still fired, but nothing happens (in this case).
Catalin Pop correctly explained that the code is, in essence, "resetting" the drop down list.
Legacy Form fields use a similar pattern - macros can fire when the user enters/exits an form field. But the design for that was you had to create a Public Sub and assign that to the form field in the Properties.

Answer 2

I think the logic here is quite simple.
Basically the code searches for a content control named Classification within the entire document.
After it finds it, it clears all of its drowdown entries - like a reset.
After the cleaning part it again searches through the entire document for all content control that start with word "Level" and it collect the text for those controls and their order in appearance.
With this info collected it then fills the dropdown optios for the classification control above. (e.g. 1 Level X, 2 Level Y.. - based on what it finds in the document for controls starting with Level in their name)