The left hand column of the edit screen show all the sections within your proposal. A section is like a chapter and you can have as few or as many as you choose. Templates often will contain a base set of sections but you can freely add/delete or even re-arrange these sections. To re-arrange a section simply drag and drop it in the new position. To add a new section click the "Add Section" button at the bottom of the screen.
One a new section has been created you can give it an appropriate title and start adding content. Content can be added manually using the "+ Insert" buttons or from the content libraries on the right. Once you have added a new section, it will be displayed in the left column. You can also navigate to each proposal section by clicking on it.
You can see these dynamic section headings in use on your proposal when you click on "Save and Preview", they automatically create the dynamic left column which is clickable to quickly navigate around the proposal.