I only know reference cards *about* UML. Where do you get it from that there should be reference cards *in* UML?
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
I have used collaboration cards also known as CRC cards. It's where you (and your team) get some pieces of card and start thinking about the candidate classes in your system and how they will interact with each other. e.g. if you identified that you needed a Personnel Officer class and an Engineer class you would create a card for each. You would then start thinking about the responsibilites of each of these classes. e.g. a Personnel Officer might be responsible for Preparing An Employment Contract for an Engineer. These responsibilities will become methods. When you can see how each class needs to interact with other classes, it gives you stronger ideas about coupling and cohesion and helps you make decisions early on about which classes need to reside in the same package etc. The cards are very useful when planning large systems where many analysts and developers will be involved in the design.