Nige, in safety terms, the IPTL's responsibility, and hence the IPT, is bounded by the Military Aircraft Release (MAR), in theory this should be underpinned by a full Safety Case, in practice there will be a lot of claims for grandfather rights etc, based on the aircraft's previous operating history.
The MAR defines the defines the build (design) standard for the aircraft, it defines how it should be maintained/serviced etc, and of course the limits to how the aircraft can be flown and operated, all this is will be defined in the Aircraft Document Set (ADS).
The Release To Service Authority (RTSA) then produces a Release To Service (RTS) based on the MAR, for a line pilot, the RTS would generally be more restrictive than the MAR. Of course there can be more than one level of RTS, for example a test pilot would be able to use more of the operational envelope. The RTSA may also define different limits for various operational reasons, perhaps be prepared to accept greater risk in certain circumstances. I don’t think this would normally mean the acceptance of greater levels of unserviceability though.
In terms of maintenance and servicing, the IPT defines how the aircraft should be maintained and serviced in the ADS. The Aircraft Operating Authority (AOA) is responsible for ensuring that the aircraft is maintained in accordance with the ADS. It’s all about maintaining that build standard, or design intent, faults are rectified and the aircraft is serviced to prevents to prevent failures. What the people on the line can do is very tightly defined by the IPT, the ADS will contain all sorts of limits, for things like fuel leaks, cracks, husbandry etc. Anything outside these limits has to be referred to higher authority, ultimately up to the IPTL who has delegated responsibility for airworthiness, a responsibility that ultimately comes from SofS.
Simple really !
S_H