Four characters are a specification — D y n 11 tells you the connection of both windings, whether the neutral is available, and by how much the low voltage lags or leads. IEC 60076-1 sets out exactly how it is written.
The alphabet — Letters are written in descending order of rated voltage, whatever direction the power actually flows.
The clock number — The HV phasor of phase I points at 12. The clock number is the hour on which the LV phase I phasor points, the sense of rotation being counter-clockwise, I - II - III. Each intermediate and LV winding carries its own number, immediately…
The ones you meet on site — The bracket notation, the +d / +y suffix and the open-winding letters are all in IEC 60076-1, 7.1.3 and 7.1.4.
Why the two symbols must match
A connection symbol is a testable statement — It is verified at works by the phase-displacement measurement, and it appears on the rating plate. If the drawing, the datasheet and the plate do not agree, one of them is wrong - and the plate is not usually the one.