Departing analog and digital sections.

Do you only need to depart high-frequency digital circuits (like IC that contains SCLK) from analog circuit, or should you depart any digital circuit from analog circuit?
Is that the whole meaning of this departing? departing high-freq. circuits from analog circuit?

Or is there anything else that i'm missing?
Because i was wondering if the separation needs to be - High-Freq. Vs. Low-Freq. - and not Digital Vs. Analog.

Thanks in advance.

I think you mean "decoupling". The analog section is more sensitive to noise on the supply lines, so you typically filter it with an RFC. Use decoupling capacitors liberally, on the digital AND analog side.