g refers to gravitational acceleration due to gravitational force. When an object moves upwards, it moves against gravity, so it decelerates at a rate of $9.81ms^-2$, meaning it accelerates at a rate of $-9.81ms^-2$. When moving downwards, it moves with gravity, so it accelerates at a rate of $9.81ms^-2$.

$g$ refers to gravitational acceleration due to gravitational force. When an object moves upwards, it moves against gravity, so it decelerates at a rate of $9.81\;\mathrm{ms^{-2}}$, meaning it accelerates at a rate of $-9.81\;\mathrm{ms^{-2}}$. When moving downwards, it moves with gravity, so it accelerates at a rate of $9.81\;\mathrm{ms^{-2}}$.