Metal: This is the preferred mode for macOS and means the user is taking full advantage of the most modern GPU APIs on that platform.This happens if you explicitly set the Technology Preference, ' Older GPU Mode Pre 2016)', or if Photoshop determines that the software renderer is required for better stability. Software: Software rendering is a Windows-only mode that returns to the Direct X 11 renderer.D3D12: This is the preferred mode for Windows and means you can take full advantage of the most modern GPU APIS on that platform.The visual interface for many features like crop, selections, and transform will drop color borders or become harder to view (thinner). CPU: CPU mode means that the GPU isn't available to Photoshop for the current document, and all features that have CPU pipelines will continue to work, but the performance from GPU optimizations will not exist so these features could be noticeably slower, such as - Neural Filters, Object Selection, Zoom/Magnify, etc.Here's the list of available GPU modes in Photoshop:

External graphics card for imac