"Column Z" is the column in the Pokémon Capabilities.xlsx spreadsheet that's titled "Pokémon you can choose". It comes after "Massive Pokémon (32 ft and above)" and before "Pokémon that have ability A."
Sorry for the confusion. "Z" is what the letter at the very top of the column is, since Excel and Google Sheets organize columns in spreadsheets by letters of the alphabet. It's the 26th column, so it's Z.