In CHOPs, Extend Conditions determine what numbers you get when you try to get a channel value that is outside its start-end range - in its Extend Regions, before the CHOP's start time or after its end time. The CHOP may hold its last value, it may cycle, repeat, or be held at a default value. You can see a CHOP's extend conditions in the info pop-up (middle-mouse click on the CHOP), or by looking at a CHOP in the channel viewer outside its start-end range. You will see that a Pattern CHOP cycles and a Keyframe CHOP holds in the Extend Regions.
You can change the Extend Conditions with an Extend CHOP or via menus on the Channel page in many CHOPs.