AS @Luke_C says you may not need a Multi-Row tool. it seems from your description and data that you don't care about "Serial No" just description. So to get a summary of values at description level a simple summary tool grouped by Customer Name and Item Description, month and Serial No become irrelevant. And simply summarize the "Revenue". This gives you the desired summary at customer and description level.
I noticed you have different serial no's and values within a common Description thus the second option below.
If you want to do this at the "Serial No" level then you have to assume where blank it relates to a non-blank row where the absolute values would match.
For this, one way is use a formula tool to create an absolute value for the negatives. The Sort by Customer (ascending), Description (ascending) and Absolute Value (ascending) and Serial no (descending). This organizes the data so the blank serial no rows will be next to the ones of same absolution value but after the ones with Serial no populated.
Then a Multi-Row Formula tools is useful to populate the missing serial no. For the Serial No field update "If Row-1 (customer & Desc and Absolute Value) are equal to current row values then [Row-1:Serial No] else [Serial-No] endif"
Now you have the serial no column populated and can summarize as before but to the level of serial no, adding Serial No to grouping, and not just description.
