I've tried to make an example and attached it here. Hope it works.
This is a simple chart. Two "circles" or "islands" that are connected by a marriage of Child Ext Fam. She is a daughter in one "circle" or "island" and is a spouse in another "circle" or "island". I have given her a color of Yellow. I have given the others in the the extended family of her spouse a color of Sand.
Note, as John pointed out, you can distinguish a chart with colors and FTM could even use lines - with this simple example. But imagine when you have 20, 30, 40 thousand people in your file. It would be a mess. In fact, even the way that FTM does it is messy. By messy, I mean a person is found in their "blood" island at pg 232 of an EFC and in their extended family island (assuming only married once) at pg 643 of the EFC. That is easily handled by the Person Locator within FTM in the lower right corner, but very hard to find in the paper report. But, I can't think of another option that can work. Is there another option?
Note that a single line is parent to child. A double line is from spouse to spouse.