Merchant Seamen are pretty much self employed in real terms, certainly from the 1800's through until very recent times (I packed it in in 1983) and within a bit could pick and choose which ships we preferred, usually by Home Trade or Foreign destination, company owning the ship, length of voyage preferred, some prefer tankers, some prefer liners, etc, etc.
Officers would serve an Apprenticeship with a company, 5 years, and then might be offered a position with the company and might well stay with a company for their full career at sea, others might change a couple of times between companies trying to gain promotion because simply holding a Master's Certificate didnt guarantee that a chap would sail as Captain until he had sufficient "sea time" experience and the luck to find a company wanting a Master. Some changed a lot.
Ratings (as I was) might crew a Tanker on an engagment Jan-April, have a couple of weeks leave and return to the "Pool" (the MN employment office in their local port) and then ship out aboard a Container ship in May-June, a bulker Aug-Oct, a Royal Fleet Auxiliary November-March, etc, etc or a passenger liner (or aboard a General cargo ship, a tramp, in days gone by).
It was common though for long in the tooth seamen who had continually managed to gain employment aboard a particular vessel/company (such as Petty Officer ratings and cooks/stewards) to get a "company contract" which virtually guaranteed them employment and regular periods of leave with the same ship or other ships owned by the company. The same thing often happens with the Royal Fleet Auxiliary.
So in short
All of them are Merchant Seamen, I sailed aboard a variety of ships including passenger liners.
