Compatibility is a non mandatory feature. It is usefull only if there are dependencies between the products of the different groups .
Product Builder is using Tags to create compatibility relations between 2 or more products. Hence the products that have at least 1 common tag between them, are considered compatible.
How is working in the front-end
'Compatibility Check' works from top to bottom. That means that when a product from the 2nd group is selected, PB will check for compatible products in the following groups (the 3rd ,the 4th etc.). The products which are not compatible with a selected product will be either disabled or hidden (depending on the settings for the incompatible products).
Lets have an example.
In the following Image we a PC builder. Now lets say that the user selects the 1st available motherboard named ASUS M2N68-AM.
PB gets the tags of the products of the current group and disables the products of the following groups that have those tags assigned, leaving enabled only those that have the same tag as the selected product.
In our example the tags of the group 'Motherboards' are AMD and intel. PB gets those 2 tags, then excludes the tags of the selected product (AMD), what left are the tags of the products that should be disabled, in our case the products with the tag intel.
More complex compatibility examples can be found in our Knowledge Base
2 or more groups should have at least 2 common tags between them, in order the compatibility to work
Products with no tags assigned are considered compatible with every selection
To use Compatibility Check, the 'Compatibility' should be enabled in that 'Custom Product's page, in the backend.
You can also let the user decide if he wants to use it, setting the "Display Compatibility" Option to yes from the pb settings and/or the menu item. Even in this case the "Compatibility" field of the used Custom Product should be enabled, to have this functionality working.
How is working in the Backend
If you intent to use compatibility, first you have to create your tags. Then go to the compatibility page and set tags to your products.
The best way to assign tags to your products is to use the Groups filter, starting from the 1st group and continuing down the last.
So first select the desired group and then press the edit button positioned at the right of your products. From the popup window , select the tags you want to assign to that product and press update. Thats it! your product now has some tags assigned.