All these words are obviously from German military usage. "Feld" means simply "field" as in "the place where battle commences", but the word can also mean "area" both in the physical sense as well as in the sense of "area of expertise".

"Feldjäger" is the German military police, "Feldkurier" would be an army messenger, "Feldwebel" and "Feldmarschall" are ranks, the former still exists today. I suppose that a "Feldsachmeister" would be someone in charge of equipment. There's also фельдшер; which in German, "Feldscher", would be a low-ranking military doctor (Feldscherer = he who cuts on the field).

I have no explanation for фельдсвязь, obviously the second half is Russian and not based on a German word.