I'm a bit surprised that there's no additional recommended field in the data format for quantities. For example, if the usage record is for a data transfer, wouldn't we want to know how much data was transferred? If it's for a Kepler workflow, wouldn't we want some record of how much work (tasks, cycles, SUs, compute hours) was done? I'm thinking this would have to be service-specific, but it's unclear how the data format would allow a service to supply that information. If it's just a matter of allowing services to add fields at their discretion, we should mention it and give an example?