Matching rules allow us to ensure that our custom questions are being asked for the right products, whether that's one single product, a whole category, or any products that have "X" in the title.
When you're adding a custom question and get to the matching part, you'll see this screen
Check out the different options available for "Property" and "Operator" as you might find some that work really well for you, some interesting ones include:
The standard option is "Product equals X" where you select a specific product from your catalog. This works well, but have you considered "Product does not equal X" as an easy way to select all of your products except for X?
If you select "Product title", you can now use more operators, like "Contains" or "Starts with" which provides a lot more flexibility
"Product category" is great if you have a category setup and want a custom question to apply to all products in that category
If you're in the middle of creating a new custom question, you'll have to click "Create" before you can do this, but if the question is already created and you're editing it, you'll see the option to "View matched products"
Clicking this will show a dialog box with the products that are matched to this question (based on your rules). Use this to confirm that the right products are being matched, otherwise, feel free to make some changes to the rules to adjust.
As you've seen, you can edit the matching rules after a custom question has been created but note: removing a product from the matching rules will not remove the question from reviews that have already been submitted with answers.
Remember to "Add" each new matching rule for them to be saved, or "Delete" any that no longer work for you.
Once you've done this, your custom questions are ready to go!