Is this a joke or serious? I know nothing about decking but want to avoid making such a heinous error in future.
tldr; it depends, confirm with supplier.
Basically decking has always had grooves in - at some point in time in the UK someone thought "this decking looks so much nicer with the grooves on the top" and "this might actually not be as slippy, because of course I haven't maintained my wood over the years, as the grooves will give extra friction".
In theory all decking should have grooves on at least one side - as the grooves do aid in ventilation, reduce moisture retention etc.
But not being a master in identifying wood I don't know the scientific properties of wood to say if grooves width and spacing has a huge impact on the life span on the board.
General rules would be;
If you have decking which has grooves on one side and a flat smooth surface on the other than it's smooth side up.
If you have decking with has grooves on one side and a flat rough surface on the other then it's grooves side up - who would want to walk on a rough splintery surface. But being flat side down it is 100% not going to last as long as a grooved side down board and would need treating before installing.
If you have decking with grooves (reeded, grooves, ribbed) on both sides then look for any manfuacturing markings / stamps - if there are then that side is down. If there are no obvious markings / stamps (they'll probably on the ends of the board) then just do what ever you want.