Usable Capacity
The real share of energy found by multiplying the amp-hour figure on the label by the depth of discharge the chemistry allows. The usable capacity of a lead-acid or AGM 100 Ah battery is roughly 50 Ah at the %50 DoD limit; the label value alone is not enough for planning.
The quantity to compare when choosing a battery is not the label capacity but the usable capacity. Two batteries with the same label offer very different real energy in different chemistries. In the lead-acid family, half the capacity is in practice considered unreachable in order to preserve life; in LiFePO4 this share is markedly higher.
The calculation is set up like this: usable capacity = label capacity × accepted DoD. If your daily consumption is 60 Ah and your chemistry is lead-acid, the required label capacity comes to roughly 120 Ah. The label capacity needed to cover the same consumption with a chemistry that allows deeper use is lower.
This distinction also changes the budget comparison. A large lead-acid bank that looks cheap can, because of its weight and limited usable share, take more out of the RV's load capacity than it is worth. If you want to supply loads such as levelling, the fridge and the pump safely, always base the plan on usable capacity.
The context in which this term is used: How to choose levelling leg capacity