DAC

In-rack / adjacent. Typically sub-3 m at 400/800G.

ToR to NIC when the geometry is short and the rear of the rack can take the copper bulk and the bend.

Avoid: Row-to-row. Stuffed chimneys on air-cooled HGX. Any run you will service weekly.

AEC

Active copper. Longer than DAC, still copper.

In-row when you need retimed copper and cannot land fiber yet. Treat power and heat as part of the cable, not as a free lunch.

Avoid: Campus. Underfloor spaghetti. Mixing AEC with untested MPO adapters on the same rail.

AOC

In-row to in-hall. Fiber with the optics already on the ends.

When you want optical reach without field-terminated MPO. Good for known lengths. Bad if you need to recut.

Avoid: A plant you will grow by splicing. AOC is a SKU, not a structured plant.

MMF (OM4 / OM5)

In-hall SR-class. MPO/MTP trunks.

Leaf to leaf inside a hall when the SR optic and the MPO count match. Polarity is a design, not a guess on site.

Avoid: CIN / campus distances. Mixing OM3 leftovers into an OM4 trunk and calling it tested.

SMF (OS2)

Hall to hall, and CIN-class. DR/FR/LR optics at 400/800G.

Spine, meet-me, storage across halls. NVIDIA's public DSX overview puts a 500 m optical reach limit on the cluster interconnect. That is a fiber plant number, not a switch SKU.

Avoid: Unspecified polarity. Unlabeled MPO-16 vs MPO-12. No OTDR on the backbone.