In the classical OBV (On-Balance Volume) indicator, it simply takes the idea from traditional tape reading - treat the "up tick" as Buy, "down tick" as Sell, and it assumes no change in price as neutral* (*which is not the case in tape reading).
When it comes to interpret the daily volume as such, errors will add up cumulatively. For example, there are days when a Doji Star with high volume just merely one cent higher than yesterday price and the whole day volume would be taken as a BUY Volume....
Here is a gentlemen, William C. Garrett, attempted to break down the daily volume into two parts in his book - "Torque Analysis of Stock Market Cycle".
Published indicator has two modes: Cumulative and Time Segmented. Time Segmented Volume (TSV) - performs a MACD operation on the Garrett Money Flow.
Note on Divergence: When using a indicator as Time Segmented Money Flow, divergence would surely occur on and off. This is where Wyckoff 3rd principle comes into play - "Effort vs Result" that is not matching. Meaning that the cumulation of shares goes in one direction while the price goes another direction.