The present document studies use cases and charging triggers for usage metering of virtualised resources. It proposes new functional blocks for: 1) the collection and provision of accounting information; and 2) the triggering of charging requests. The interfaces (and information flows) between the proposed functional blocks and the current NFV Architectural Framework are part of the study. The following models have been taken into account: Infrastructure as a Service (IaaS)@ and VNF as a Service (VNFaaS). The present document includes recommendations to either modify existing or new specifications@ or both. While management and orchestration event charging for VNFaasS is part of the present work@ usage event charging for VNFaaS is for further study.