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An agent 

• unique identifier (identity)
• type (individual or group; if group, what type of group, e.g. organisation, company or movement; auditor)

An activity

• unique identifier
• location
• start time
• end time (possibly)
• agent
• inputs (e.g. materials and energy; consumables and non-consumables)
• outputs
• processes
• objective (create, reduce, destroy, avoid) (optional / not required in all instances)

Should there be an event class? An event is like an activity but without the intention of an agent. An accident is an event but it is not an activity. 

A state

• unique identifier
• location
• point in time
• indicators (metrics)
• type (actual [measured] or baseline [counterfactual])

A measurement instruments (observations)

An audit (the process / action of verifying an impact claim)

• unique identifier
• auditor ID
• impact claim ID
• date

A standard

• unique identifier
• applicability - type of agent, type of activity, state
• requirements (e.g. reporting requirements, calculation requirements)
• version

An indicator

• definition
• unit of measure
• rationale
• calculation
• credibility

An impact claim (a state change claim)

• unique identifier
• unit of measure
• magnitude
• state "before"
• state "after"
• causal connection between agent and activity
• standard followed (ID)
• effect (e.g. primary or secondary)