Plus Minus function allows you to create 'Nudge Rules' which react to the '+' or '-' keys being pressed in numeric cells.
You can create a 'default' Nudge Value for a whole column, or a 'custom' Nudge Value which will increment / decrement based on other cells in the row.
In this example we created 2 Plus Minus Rules: 
a default one for the 'Item Cost' column of 10
a custom rule for the same column which will increment / decrement by 20 if Employee is 'Janet Leverling