With some airlines in some countries (generally small companies without unions) there is no seniority... or say, there is a seniority in the "captain" group, and another seniority in the "first officer" group. Then it is possible that, if in need of additional captains, company hires direct entry, or selects first officer to become captain by seniority, qualifications or merit.
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With PanAm, we had a pilot seniority list, with the ALPA union.
However professional flight engineers (PFE) had their own list, with FEIA union.
After 1963, newly hired F/Es were generally pilots (a CPL/IR was required) and were on the pilot seniority list. but PFE were non-pilots, with F/E licence.
Some airlines still have separate F/E seniority lists... as long as there will be airplanes requiring F/Es.
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Happy contrails