Why doesn't the iMac Pro (2017) support the Apple Pro Display XDR at 6k resolution? Apple's Set up and use Apple Pro Display XDR web page does not list the iMac Pro as a supported computer in this section:


  Pro Display XDR supports a resolution of 6016 x 3384 with 10bpc on
  these Mac models:


However, it does list as supported:


  Mac computers with Thunderbolt 3 ports connected to Blackmagic eGPU or
  Blackmagic eGPU Pro


Apple's Use Blackmagic eGPU with your Mac web page states:


  Blackmagic eGPU includes a Radeon Pro 580 graphics processor with 8GB
  DDR5 memory, giving your connected Mac graphics performance that's
  similar to a high-end 27-inch iMac.
  
  Blackmagic eGPU Pro includes a Radeon RX Vega 56 graphics processor
  with 8GB HBM2 memory. This gives your connected Mac graphics
  performance that's similar to iMac Pro.


iMac Pro, with any of its three built-in graphics options:


Simultaneously supports full native resolution [5120‑by‑2880 (5K)] on
the built-in display at 1 billion colors and:
Two 5120‑by‑2880 (5K) external displays at 60Hz with support for
1 billion colors, or
Four 3840-by-2160 (4K UHD) external displays at 60Hz with support for
1 billion colors, or...


Why is an additional external GPU with the same or lesser capabilities as the built-in iMac Pro GPU needed in order to drive a single external Pro Display XDR at 6k?
For 6K to work, you need one of the following:


A GPU that supports one DisplayPort 1.4 connection at HBR2 speed using Display Stream Compression (DSC). The signal can be sent directly using USB-C, or as Thunderbolt.
A GPU that supports two DisplayPort 1.4 connections at HBR3 speed without DSC. The GPU needs to be connected to a Titan Ridge Thunderbolt controller that can transmit both of those DisplayPort 1.4 singals over Thunderbolt.


The iMac Pro has a GPU that can support 6K without DSC, but it has an Alpine Ridge Thunderbolt controller that can only transmit DisplayPort 1.2 signals (but if it can't transmit DisplayPort 1.4, then how does the iMac Pro transmit 3840x2160 at 60Hz with the USB-C Digital AV Multiport Adapter when the adapter only has two DisplayPort 1.4 lanes?).

GPUs that support DSC are Nvidia RTX (Windows only), AMD 5300M, 5500M, 5700XT, W5700X, W5700. The W5700 has a USB-C port but I don't know if it works in macOS. The RTX cards have a USB-C port which allows 6K to work in Windows.

The Blackmagic eGPU doesn't support DSC, but it has a GPU that supports two DisplayPort 1.4 connections at HBR3 speed. The GPU's outputs are connected internally to the Titan Ridge chip in the Blackmagic eGPU which transmits the DisplayPort data over Thunderbolt to the Thunderbolt display that is connected to the eGPU. However, support for dual HBR3 connection (for 6K) requires a firmware update that was recently released by Blackmagic. Otherwise, it only supports dual HBR2 (allows only 5K).

Other computers that have a GPU that doesn't support DSC but does support dual DisplayPort 1.4 might be able to connect to the XDR display using a GC-TITAN RIDGE to achieve 6K. However, this might have a problem similar to the Blackmagic eGPU and therefore might require a firmware update that might not exist yet and might not ever exist. When I try to connect two HBR3 displays to the same Thunderbolt port of a GC-TITAN RIDGE, one connects at only HBR speed. I have not tried the firmware update from October. I doubt that it addresses this problem.

I do not know if the EDID of the XDR display contains enough information for dual HBR3 tiled mode to work in other OSs besides macOS. Specifically, the EDIDs that I have do not contain a 3008x3384 timing even though they specify a tile of that dimension. This might be because I've only seen EDIDs for dual HBR2 5K and HBR2 DSC 6K modes. The missing timing info exists in the mtdd files for the XDR display at /System/Library/Displays/Contents/Resources/Overrides/DisplayVendorID-610
where DisplayProductID is one of ae21, ae22, ae23, ae2d, ae2e, ae2f.

To obtain DisplayPort connection info, use the following command:
/System/Library/Extensions/AppleGraphicsControl.kext/Contents/MacOS/AGDCDiagnose -a &gt; AGDCDiagnose_a.txt 2&gt;&amp;1

There is also a command specifically created for the XDR display
/System/Library/Extensions/AppleGraphicsControl.kext/Contents/MacOS/DisplayDiagnose -a &gt; DisplayDiagnose.txt 2&gt;&amp;1