A 10 A lighting circuit in a home can take 20 lighting points, which sum to 10 A under AS/NZS 3000:2018 Table C9. The tool checks that the sum stays within the circuit-breaker rating.
The count
In a domestic installation, one lighting point counts 0.5 A. A 21st point takes the sum to 10.5 A, over the rating, and the tool reports a fail.
Socket‑outlets on a lighting circuit
Table C9 puts no socket‑outlet on a conductor under 2.5 mm², bar one counted as a lighting point. On a 1.5 mm² lighting circuit the tool refuses a socket‑outlet for that reason.
See also: What size cable for a lighting circuit in NZ? and Minimum cable size for a lighting circuit.