The abbreviation n.b. is the modern English version of the Latin phrase nota bene (note well, or take good notice). This is used in modern textbooks to emphasise a particular point.
The word limnum doesn't occur in Latin. There are a number of Greek words beginning with "limn-" having to do with pools of standing water, marshes, lakes, etc., but limnum is not among them.
Cui is the dative singular, all genders, of the relative pronoun qui, quae, quod and also of the interrogative pronoun quis, quid. It means "to whom" or "to which."