The total, (or gross) capacity of a battery pack is the amount of energy it can store when it's fully charged to 100% The usable capacity is how much energy the user is allowed to access. The usable capacity vs. the total capacity can vary depending on the battery chemistry as some types of lithium-ion batteries are better suited to be charged to 100%, while others will degrade faster if the battery is frequently charged to 100%
Therefore, EV manufacturers all have different policies on how much battery they hold in reserve as a "buffer" to prolong the life of the battery. The energy in that buffer isn't accessible to the user. For instance, the Volkswagen ID.4 has an 82 kWh battery, however, only 77 kWh is accessible. Volkswagen holds a 5 kWh buffer in an effort to extend the battery life.
This article has a detailed explanation based on some flack that was caused regarding the battery size of the new Lucid. It explains how the charging session used a lot more power than the expected battery capacity.