It is the unit that gets priced, billed and recorded. The gateway validates your arguments against the input schema the author declared, calls the tool, then validates the result against the output schema. One tool call produces one receipt, whether it succeeded, failed or was refused.
A tool call carries more than arguments. The gateway adds the ceiling for this run, and a signed download link for each file handle in the input. The agent can report back the units it consumed, which is what a per-unit price is computed on.
Example: one call of pdf_tables with a file handle returns the tables as typed rows and bills 0.004 EUR. A call that times out returns nothing and costs nothing.