Seniority means different things depending on an individual's expectations. One guy may be more senior at base xx but joined the company after another guy at base yy that had more experience. In general the simplest form is by company join date but temporary postings and vague promises raise people's expectations artificially. Whilst it appears NAS are blatantly abusing the contracts on the surface it may well transpire the individuals concerned did not fully understand their positions. Now a permanent contract that gets massaged into a 6 month on 6 month off is another matter and perhaps NAS can take a leaf from the Ryanair book and offer meaningful permanent contracts directly with the company with the associated benefits. Ryanair seemed to have learnt a lesson from the exodus and now concentrate on pilot retention not alienation which appears to be the NAS policy