If you look at the DDR3 1333 Qualified Vendors List (QVL) list for the Asus Z87-A motherboard:

http://dlcdnet.asus.com/pub/ASUS/mb/LGA1150/Z87-A/Z87-A_DRAM_QVL_0730.pdf

You can see a column called "DIMM socket support (Optional)":

1. What does this mean?
2. What does this specifically mean for a memory that only has a dot in the "2 DIMM" column?