I have a road frame on order. It calls for a BB86 Press Fit bottom bracket. The SRAM chart shows that a 3mm spacer is needed for their DUB Press Fit 86.5mm bottom bracket. I have purchased their bottom bracket but it did not come with a 3mm spacer. I contacted SRAM and they say it is needed, but I was told by a repetable source that the spacer is only required if the BB shell is narrower than 86mm. Also, 86.5mm and 86mm mean the same thing. Can someone clarify if a spacer is needed, and why?

I’d say yes and suspect it’s a 90mm long spindle. Edit: no clearly that’s nonsense as it also needs to be long enough to engage the drive side crank arm. It’s 114mm apparently.

If the bike takes a bb86 by the way, these are internal bearings I think? Those 30mm (29mm?) DUB spindles don’t leave a lot of room for bearings so pick up a 2nd BB while you’re at it. It probably won’t last too long.

If you don't have a gap between your crank arm and the bearing, then you are probably good. Otherwise you need the spacer to take up that gap. Gaps are not desirable for two piece cranks.

I have installed two SRAM DUB English threaded bottom brackets and both came with a variety of spacers. For press fits, I have always used Token thread-together bottom brackets..They also included several spacers of varying widths. I install at the recommended torque with minimal spacers and add them until there is no longer any side to side play. If your crankset has an adjusting ring, then add spacers until you can eliminate side to side play by rotating the adjusting ring no more than one or two turns. The chart you included is accurate if you measure your BB shell. You can purchase DUB spacers from your LBS or online.