Vent Clearances

Hey Guys-

The attached PDF is taken out of a 92% furnace installation manual. On the diagram from the first page, what is the difference between a mechanical draft vent terminal and a direct vent terminal?

The mechanical draft (conventional vent 1-pipe furnace on page 2) shows the clearance from openings at 4’. The direct vent (direct vent 2-pipe furnace on page 2) shows the clearances from openings at only 12" (for 50k+ btu).

Is the 4’ mechanical draft clearance for appliances like 80% furnaces, typical water heaters, and gas fireplaces? And the 12" direct vent clearance for appliances like high efficiency furnace vents?

So the PVC vent/intake pipes on a high efficiency furnace only need to be 12" from an operable window or door?