Explain the units thing?

Every plumbing fixture has an equivalent fixture unit rating, which is what you are looking at. Length of piping does not have a relationship to units except that the longer the pipe, the fewer units it can handle in some cases. Your code book will have a table which gives those fixture unit equivalents. A lavatory is the basic standard at one unit. Bathtubs will be 2 or 3 depending on where you are located. Toilets will be 6 units, etc..

A FU ( fixture unit) is a measurement plumbers eng etc size waste and vent piping. A fixture unit is 1 cu ft of water (7.48 gallons) and this FU is timed going down a drain pipe. For
example a basin is ONE FIXTURE UNIT as it contains a max of 7.48 gallons and it will empty ideally in one min. Then you have roof drainage Square feet which normally is 3 Sq Ft = 1 FU after the normal sizing table is used,
AND the pitch of the roof which predicates the leader sizing, more pitch more FU’s

Again depending on local codes and rain fall conditions based on a 10 year chart.

The only time you have to concern yourself with FU in developed lenth is for vent piping, as for waste or soil lines you base the FU on the pitch of the pipe and diameter.

The Vertical is based on the soil/waste stack FU chart NOTHING to do with developed lenth.

If your sizing water lines then you must figure the total fixture demand and again as per local codes size your water lines accordingly. (cold water lines normally larger)

Water lines are a little more complex as you have friction losses to calculate in finding the proper FU peak demand as well as hydrostatic losses (.434 height and to figure how high a certain pressure will elevate water you take 2.31 PSI will give you an approx height the water will go LESS FRICTION LOSSES for developed lenth of piping and fittings and valves

When sizing a pump ( sump pump/ sewerage ejector) discharge a lot of codes calculate ONE GPM as 1 FU.

The problem with over sizing the drainage/soil/ waste piping is erosion of the piping and not getting the proper scouring action needed to keep build up to a min.
Too much velocity on the horziontal will cause excessive piping erosion on ferrious and non ferrious metal piping systems PLUS the noise factor (especially on garbage plastic systems)

The vent piping can be oversized with no adversed condition.

Remember one thing about “plumbing” the final responsibily is the licensed master plumber for the proper pipe sizing NOT the engineer or the
architech. If the system is wrong then the plumber has to face full responsibility and rightfully so.

This seperates the REAL PLUMBERS from the dabblers and bums out there. Have fun