Put Bull Spreads can also be called Short Put Spreads and are made up of long put option and a short put option in the same expiration month but where the short put has a higher strike price than the long. The term "short" would be used in reference to the premium received, which is a net credit, NOT because of the underlying directional bias.
The Max Loss is limited to the difference between the two strike prices minus the net premium received for the position.
The Max Gain is Limited to the net credit received for the spread. I.e. the premium receieved for the short option less the premium paid for the long option.
When to use: When you are bullish on market direction.
A Put Bull Spread has the same payoff as the Call Bull Spread except the contracts used are put options instead of call options. Even though bullish, a trader may decide to place a put spread instead of a call spread because the risk/reward profile may be more favourable. This may be the if the ITM call options have a higher implied volatility than the OTM put options. In this case, a call spread would be more expensive to initiate and hence the trader might prefer the lower cost option of a put spread.