Not quite store - simply, whatever you don't use in the previous month, half of that will be added to your regular allowance every month.
For example, if your allowance in month 1 is 1TB and you use 500GB, you would get half of that (250GB) added to month 2's regular allowance, making for a total of 1.25TB. If you again only use 500GB of this, in month 3 you would have 1.375TB and so on.
In other words, only half rolls over and you can't "store" it, but you can continually increase your allowance to up to 2 times your normal quota, if you don't use very much.
In other words, it helps to smooth out the peaks and troughs of regular use (e.g. holidays, visitors, large OS/game updates).
I for one will gain a lot from this and the recent 50GB addon, as instead of having a fixed 150GB that, if exceeded, would immediately cost £10 for 50GB that are then lost at the end of the month, I now have at least 200GB, which if I exceed I have to pay £10 but get to keep that until I next need it, and if I use less I can save up for next month.
I was on the brink of considering cancelling AAISP and switching to "unlimited" but this may just have saved them
Huawei HG612 VDSL2 modem. Linksys WRT1900ACS router. AAISP Home::1 80/20 150GB + copper pair on TTB (by request). Sync: 79987/20000 kbit/s, attainable ~91-94000/31-32000 kbit/s via ~180m of 1985 copper to ECI Cabinet 14, Milford Exchange (WMMFD)
Wanadoo ~2Mb (Orange takeover) > Freedom2Surf ~4Mb (TalkTalk takeover) > Virgin Media 10->30->50->70Mb (unnecessary second line as work reqd BT) > BT Inf 2 80Mb (04/17 rises, no retention offer) > Virgin Media 100Mb (awful pings
) > AAISP 80Mb