Monarch is not able to share data between records. Though the Table looks like a spreadsheet, and accordingly we almost intuitively want to be want to be able to write formulas for calculated fields similar to Excel's referencing techniques, we can't.
The Table is really just a single database table. Each row has no "knowledge" of the others, with the notable exception being the "Leave blank" and "Copy value from previous record" option in the field properties dialog.
The solution to challenges such as yours are in handling it as a database situation:
The key value is the kicker. While it's often possible to derive a useful key, at times you have to be a bit creative and choose from Monarch functions to help you out.
This is part where I ask you to post a representative sample of what you're working with.
I have to say that from your description I would have suggested the 'fill field from previous record' option mentioned by Kruncher or looked for a way to work the required description into the table using an appended template - but as Kruncher observes without a representative sample of the report both ideas are somewhat speculative.
Two step or multi-step processes are also options I guess.