Many fields link to other fields and forms throughout the return. This is indicated by a green underline in the field, and these fields generally cannot be typed into manually.
Besides a green underline you can also see that a field is linked if there is a gray arrow next to it when highlighted. Click this gray arrow and there will be a page symbol with a change link next to it indicating a link exists or can be created.
If you click the link icon or press the link hotkey (F9) it will open the New/Existing Links window. Use this window to identify existing forms that are linked to this field or create new new links. For example this method is used to link the Sch E. Depreciation line to 4562 forms for the purposes of Depreciation/Section 179.