There is a feature in AX2012 that allows you to setup enterprise portal and use the Azure access control as a middle man to your identity providers. This was handy for the external portals that we have with AX2012. I’ve posted about this in the past on how to set this up.
It’s worth noting the announcement that was published at the beginning of April about the restrictions on creating new ACS namespaces as of June 30th 2017.
ACS had some capabilities that have now moved into Azure AD. You can read more about it here
What this means if you are implementing AX2012 R3 and intend to use the vendor portal with ACS is to have setup your environment before the cut-off but long term investigate the move to Azure AD to replace what ACS is doing in this scenario. Having said this there isn’t an official support statement for the change at this stage.