The top auto is not needed if you have not previously set a position for top. I added it to ensure that you had not left in place the top:100% which would mean the menu would be at top:100% and bottom:100% which would over-constrain the positioning and result in bottom being ignored.
I this case auto just means no position for that property.
With absolute positioning you can use top, left, right and bottom at the same time to describe the size of an element but if at any time the dimension is oner-constrained then the browser will ignore one of the values. (e.g if you said top:200px and bottom:200px but the element is only 200px tall then bottom would be ignored. However if the element was 600px tall you would get a 200px tall box in the middle.)