I suggest you look up Oxford Dictionaries or something like that. The pieces you cite are clearly legal papers: so you might also try googling "legal meaning of Re" or something like that.
In fact, Dictionary.com has a fair definition of the word.
...Re2 preposition Chiefly Law and Commerce.
in the case of; with reference to; in re.
ORIGIN OF Re2
1700–10; < Latin rē (in the) matter, affair, thing (ablative of rēs)
I do not think the explanation is quite right, though. It has the correct origin, from the Latin word Res, meaning a thing or, in the law, a matter (meaning the particular case being dealt with). It is not a strictly preposition, however, although it is, in a sense being used prepositionally. The whole legal Latin phrase is in re, meaning in the matter of. Re is, as the definition says, the ablative case of res. From there, the phrase in re is often use in shortened form re meaning about or concerning. The word Re is frequently used in the heading of formal correspondence to indicate the topic to be covered in in the letter. So you can have:-
Re Your Letter of May 16th; or
Re Purchase of Flat Iron Building NYC
Going back to the first document, the letter 'D' refers to one of the persons involved in the legal case being discussed, who is not being mentioned by name, presumably to protect anonymity.
So, "Re - D" is brief a way of saying: "Concerning D" (whoever D is - presumably known to the relevant lawyers.)