Full Idea
In epistemic logic we read Υ as 'KaP: a knows that P', and ◊ as 'PaP: it is possible, for all a knows, that P' (a is an individual). For belief we read them as 'BaP: a believes that P' and 'CaP: compatible with everything a believes that P'.
Gist of Idea
Read epistemic box as 'a knows/believes P' and diamond as 'for all a knows/believes, P'
Source
M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.11)
Book Reference
Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.28
A Reaction
[scripted capitals and subscripts are involved] Hintikka 1962 is the source of this. Fitting and Mendelsohn prefer □ to read 'a is entitled to know P', rather than 'a knows that P'.