I checked the 2018.4 Editing Guide and the picture of the Markers windows on page 420 doesn't have a Part column.  What is the Part column (between the Track and Comment columns)?

Spanned Markers.
Spanned Markers are used to show 'parts' of a timeline - usually the 'content', as opposed to the lineup signals and black between segments/acts or 'parts'.

Create a Spanned Marker and you'll see. The 'Part' numbers will be the order in the timeline, not the order in which the Parts are created. The Parts are Metadata that can be exported, such as with AS-11 files.

N Ryan:

Spanned Markers.
Spanned Markers are used to show 'parts' of a timeline - usually the 'content', as opposed to the lineup signals and black between segments/acts or 'parts'.

Create a Spanned Marker and you'll see. The 'Part' numbers will be the order in the timeline, not the order in which the Parts are created. The Parts are Metadata that can be exported, such as with AS-11 files.

Cool.  I guess Spanned Markers can't be added on the fly like regular Markers.

chuckkahn:

I guess Spanned Markers can't be added on the fly like regular Markers.

They're special Markers. And they require a process to create them (Mark In, Mark Out, Create Spanned Marker) so no, you can't create them on the fly. And because of the level of precision required at the times you would use them, I'm not sure you'd need/want to create them on the fly. Different coloured Markers should take care of those needs.

Enjoy.