I’ve spoken a few times about working on modularising PBBooking as part of v4. One of the areas I have just completed working on modularising has been the validation methods.
As of v4 validation will move from a select box under the Validation Settings tab.

Instead this will be replaced with separate validation plugins available from the Plugin Manager of type Pbbooking.

Although this makes a couple of extra clicks and gives two different places to configure, I feel the benefits make this worthwhile. There’s a couple of reasons that I wanted to do this.
Firstly, moving validations into plugins will give people with javascript and php knowledge the ability to write custom validation workflows without having to hack the core of PBBooking. Do you want to only allow a client to book a followup after they’ve had an initial? Well using plugins means you can write a plugin to do that.
Secondly, moving validations into plugins means that you can set different validation workflows based on the Joomla ACL settings. Perhaps you want to allow auto validation for clients that have an account on your website but require email validation or admin validation for users without an account. The plugins allow you to do that.

As in the current releases the free version will include the client validation workflow. Auto validation and admin validation will be shipped with the subscriber version.