The reason that the manufactures are a bit vague on answering your question is because there really is no finite answer. In order to get a finite answer you would need to check your local building code because the regulations can change dramatically from one code to another.

First your code will define whether you need standard single wall or double wall B-vent pipe.

Typically you may have a horizontal run providing the pipe is pitched at 1/4" per foot upwards toward the stack.

You local code will limit the length of the horizontal run. Some will allow the horizontal run to be equal to or not more than the length of the vertical rise while other codes may reduce that to 60% or 75% of the vertical run.

Understanding that the vertical riser may be hot and could present a risk of injury to people on the deck some codes will not permit the riser to be within 8" to 10" horizontal of a deck surface.

The horizontal pipe must connect to the vertical stack by means of a Tee into the side of the stack and the bottom of the stack must be open to permit air to enter the stack from the bottom.

The top of the stack must be a minimum of 2' above the roof of the structure and in regions subject to snow they may require the top to be 2' above the average snowfall depth.

In addition to the normal clearances from vertical surfaces, windows and doors most codes also require a minimum height above a surface used as a deck or walkway.

Tnx, Lazypup.
Most tankless heaters have a power vent and allow horizontal runs, specifying 1/4 inch sloping down. In my case, the horizontal run will be one foot since It mounts inside and vents thru a wall. Inside is above grade and NOT a living space, under the living space but not a basment.
The county requires me to follow the manufacturer's installation indtructions, which are vague. They have no specific rule. So, I'd hoped for some justification that I could use to convince my inspector/plan checker that I can use any tankless brand and not just one that seems to allow it in their literature. Bosch looks easy and Rheem looks like a problem. Noritz, Paloma, Takagi are maybes and Bosch is not my first choice.
Al

generally, if you have 3' of clearance from the underside of the deck you should be OK. The vent limits in your ownersmanual may not say "under a deck" but there should be something. The engineering dept.s for the water heaters continously change this stuff too, so you could call them directly and ask them. Generally they are 12" above grade, 12" from a window or door, 3' above/below or 10' away from another power vent, 3' below a deck or vented beyond the deck, etc. Usually as you said, power vent/ direct vent units are considered code correct when installed according to manufacturers instructions.