Zend Framework and the Use At Will architecture

I was doing some talking with some gentlemen last night over several adult beverages and fine tobacco products. They worked for a partner of ours and had brought up an interesting point concerning Zend Framework. It seems as though people who ask them about their product set and how it fits in with PHP were asking about Zend Framework and whether or not you had to use the whole framework to be able to integrate with their software.