"Is there a way to get the Categories column field to autofill, the same way Description column ahead of it does?"
What way is that? My guess is you are talking about Numbers 'auto complete feature, which provides suggestion of what you could be entering in the cell, based on what you have in other cells in that column.
If that's the case, all you need to do is delete the content (and Pop-up- Menu format) from the cells in the Categories column, and treat that column in the same manner as you do Descriptions.
The Category labels are on a pop-up menu to ensure that each transaction is assigned to a named category, that the category name is not susceptible to typos, and that the category matches one of the categories listed on the Summary by Category table elsewhere in the document.
You can easily edit the pop-up menu cells to fit your categories list. The easiest way is to go to the Summary table and edit the Categories list in column A of that table to match the categories you want to use. When the list is complete, select all of the cells containing the list, click the Format brush, and set the cells' data format to Pop-up list, Then change the Start with menu item below the list to Start with Blank. he exit th Inspector.
Each of the selected cells is now a popup menu cell containing the same list of choices, each set to a different choice.
Click on the first cell and use the control that appears to its right to set it to 'none', then Copy this cell. (and reset it to the original value.
On your main table, choose every cell in the Category column, then Paste. Your menu is now in every row of Category
AUTO SELECTION
A formula based solution could be used to automatically select the correct category and insert it in the Category cell on each row.
The key to this solution would be a complete list of all possible outgo receivers and income sources and the category assigned to each.
Receivers to whom payments may be in several categories would require 'tags' attached to their names to express which category applies to each expenditure.
Once that is available as a two column list on a separate table, a VLOOKUP or MATCH/INDEX formula can be used to look up the receiver or source and return the proper category to the category cell containing that copy of the formula with no user action required in the Category column.
Regards,
Barry