What does 'Max allocated RAM size' mean on this mobo?

Solution 1:

On older boards way-back-when which only supported 2GB RAM in total this sort of thing referred to what you would expect to see after on-board video hardware and other devices and consumed some.

On a modern board like this (which supports 64GB as the other specs state) this refers to what memory can be allocated to the on-board GPU - so you might put 8GB in there but only see ~6.25 GB available to OS/apps/games as the GPU has claimed the remainder for its work. You usually have some control over what the GPU does claim, and of course if you completely replace its function with an add-on card (which you likely will if building a machine for gaming) it should take 0.

This information is normally listed with or directly after the name/model of GPU in a specs sheet, I assume that line has been missed out by mistake in this case (either a mistake on the seller's part or due to them copying a bad list from the manufacturer's blurb) leading to a little confusion.

Solution 2:

I believe this is related to allocating memory to on-board or on-chip (haswell etc.) GPUs which share system memory. The figure relates to the largest amount of system memory you can allocate across as extra memory for the GPU, in addition to any on-chip or on-board GFX memory.