If you mean an up arrow as well as a down arrow, this means it can increase *or* decrease the blood sugar levels. Usually this means it will affect different people in different ways, but sometimes it can mean that it will affect the same person in different ways at different times.

For example, most people find that exercise will decrease their blood sugars, but some people have reported that intense exercise causes their levels to rise.

When I go for a walk, usually my blood sugar immediately jumps up slightly and then after 10 or 15 minutes it starts to fall. After 30 minutes I am back to where I started and then my levels continue to fall after that.

https://diatribe.org/42factorsexplained gives more details about each of the 42 factors.