ATX12V is 4-pin, EPS12V is 4-pin or 8-pin ... many PSUs use "hybrid" 4+4-pin connectors for full compatibility with both form factors, mostly because mobos which only require 4-pin sometimes lack enough physical space to accommodate a larger connector.
If it's a 4+4 variety of connector then the plastic blocks are usually designed to somehow key or interlock with each other, and of course to only plug into the mobo "one way" with correct orientations. (Unless the PSU is very cheap low-end no name junk which fails to even meet basic compliance.)
Read the manual/specs carefully if you can't figure out exactly which output to use, the 12V CPU power output is often purpose-dedicated (on an isolated "rail") for tighter regulation and stability. Better to not just plug any random 12V output into it (even if the modular cables happen to fit) unless your PSU documentation says this feature is specifically supported.https://forum.digikey.com/t/common-computer-power-supply-connectors/328
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