An extended double crochet (ext-dc) is a dc st that is made taller (extended) by a chain at the bottom of the post (the vertical body of the stitch). Chain and slip stitches don’t have height so they cant be extended, but any other stitch that has a vertical height (a “post”) can.
Here is the stitch instruction:
extended double crochet (ext-dc) Insert hook in designated st/sp, yoh, pull up a loop, yoh, pull through 1 loop, yoh, pull through rem 2 loops on hook
