[Exolent[jNr]]

Adding on to what Alka stated, if used in an if() statement:

if( is_user_outside(client) )
{
    // returned the distance to the distance from the player's origin to the sky
    // if the player's origin can be traced to the sky without touching anything,
    // then a distance will be returned instead of 0, which returns true
    // therefore, this block of code would be for if player is not under anything
}
else
{
    // there is no distance returned because player is underneath something
    // this is because while the origin was traced to the sky, it hit something
    // which means player is under something
    // therefore, this block of code would be for if player is underneath something
}