[soah]

For betting a draw to be a value bet, you must have enough callers to make the bet positive EV. For example, if you think you'll win the hand one out of three times (nut flush draw), and there are eight people in the pot, you should bet for value even though you expect to lose the pot most of the time. You expect to lose one bet two out of three times, but win seven extra bets the third time.

By definition, a semi-bluff is a bet that hopes everyone will fold. Value betting your draw as opposed to semi-bluffing with it depends on how many callers you expect to get.

On the river, you usually bet either in hopes that a better hand will fold, or in hopes that a weaker hand will call. The former is a bluff, the latter is a value bet.