IMHO, this cell is exist but don't used in current time.
Sections in ShapeSheet have structure like table. Columns ActionPrompt and ActionHelp was added in Action section
as first time. Another columns added in same time or later. In Visio 2002, these columns obsolete.
MS for keep compability, saved these columns and their positions. But make it invisible for users view

These cells available only via VBA.