If I take a mate’s son and daughter for a flight and I meet the entirety of the direct costs of the flight, that is not in any circumstances a ‘cost-sharing’ flight.
Makes sense as the flight is just a straight forward private operation, the Pilot or Operator bears 100% of the costs with no contributions. There is no seat limitation as you are paying for the whole thing so can't run a quasi air service operation under the guise of cost sharing.
If, instead, my mate’s son and daughter each give me 50 cents out of their piggy banks to pay a tiny fraction of the fuel costs after I refuel after the flight, and I meet the rest of the direct costs, that is a flight that can be a ‘cost-sharing’ flight. The $1 is ‘reward’.
Yes, because it fits in with the allowable range of a cost sharing flight as stated above, so is a private operation as well being allowed to cost share.
If, instead, my mate gives me $1 at the fuel bowser, that is not in any circumstances a ‘cost-sharing’ flight. The $1 is ‘remuneration’ of the PIC.
Yes, as the person is not occupying a seat or such to make it a cost sharing flight, again restricting the circumstances that a person can take advantage of the law for financial gain. Is it important to draw that line, well yes, in CASAs eyes it limits ways you can get around the hire/reward aspects. I think It's pretty clear that the cost sharing aspect is entirely in law to allow a small group to share the cost of a flight for personal transport for pleasure or recreation or such. You start allowing third parties to contribute up to 5/6th of the flight for freight or such and it starts to run into realms where a PPL can run a quasi air service operation at a slight loss to compete with established operators. So it's not nec a safety question, rather than an operator protection, although it is in part safety as the pilot and aircraft are operating in a lower category of requirements.
And, at least so far as the subject of this thread is concerned, I’m trying finally to kill, cremate and bury the folklore to the effect that there is, and always was, a 6 POB cap on private operations. CASA has already agreed, for example, that the PIC of my 9 POB ‘charity airline’ can be a PPL.
There is no such rule in Australia, and has not been for as long as I have been flying, only a cap on cost sharing ability. We used to run Chieftain cost shares under our charter AoC, each person paid equal, but it was run as an air service operation, the pilot collected the money and paid for the charter of himself and the aircraft at a discounted rate. The pilot could have just hired the aircraft and paid full costs, but not accepted any reward to make it a private operation.