I've discovered that Process properties can 'behave' as being shareable across processes but in fact they aren't
I created two processes A an B in separate folders Folder-A, and Folder-B that read and write to a single Persistent Process Property
In Process A I created a Persistent Process Property A-Prop in Folder-A
Property A-Prop is correctly listed in the component explorer in Folder-A
Process A updates A-Prop with the current date-time, then reads A-Prop into a Notify shape.
I deploy Execute Process A
Process A properly updates and persists A-Prop as verified by examining process properties for deployed Process A, and the process log.
In Process B reads A-Prop into a Notify Shape, writes to Propery-A with Properties shape and then reads Prop-A into a Notify shape.
It appears that any Process property can be selected whether it's in semantic scope or not when developing a process!
Process B is deployed and executed
Process B reads Prop-A
It has no value as expressed in the process log
Process B writes to Prop-A using a set properties shape without error
Process B then reads Prop-A into a notify shape and it has the value written above as ex[pressed in the process log.
I examine the process properties for Process B and Prop-A exists!
No where in the component explorer does an additional Prop-A 'declaration' exist for Process B.
Boomi evidently automatically created a ghost persistent property Prop-A associated with Process B.
To sum up, Boomi allows you to syntactically share properties, Boomi automatically creates hidden ghost properties that can be read a written, the property that was selected to be shared isn't
This is a nasty bug.
Properties should be shareable across processes. It would make sense to have global local scope setting on properties so that shared properties are obvious upon inspection and prevent un-shared properties from being shared. The user is aware of using a property across processes because to do so they can, and must, navigate the component explorer to select it. Unfortunately the full path to the Process property selected is not reflected in the shape where it is being used after being selected, further obscuring it's full semantic definition.
If Process properties aren't shareable then users must not be able to select or use process properties that aren't shareable and most importantly Boomi must not not auto create ghost process properties.