The answer to this question lies in whether or not you have a dual sided board. The way to tell is if both sides are dressed with edges chamfered. If that’s the case which way you lay your deck board is entirely up to you; whichever is your preference. The alternative side to the grooved side will often be ribbed or smooth.
If the opposite side to the grooved side is clearly not meant to be seen i.e. it’s not properly finished or has no decorative features like ribbing, then clearly the decking should be fitted with the grooved side up.
Grooves are designed to channel water away from the deck and as such should be fitted with a fall in the direction of the grooves. Any fixings should be fitted at quarter points within the bottom of a grove which also makes them less visible. Contrary to some people’s belief, the grooves are not intended to provide improved grip underfoot.
Grooved deck boards are also referred to as castellated. Other styles exist such as ribbed - also referred to as reeded.