From the official site, it says "The Hot Spot Analysis tool calculates the Getis-Ord Gi* statistic (pronounced G-i-star) for each feature in a dataset" and "xj is the attribute value for feature j".
Say the entire map is divided into 100 smaller grid units, but only 50 of these 100 units have an input value. Do I have 100 or 50 "features"? For the units that don't have an input value, do they also go through the G* statistics? If so, does it mean it automatically assumes the value of 0?