The initial known integrations should be captured on the Digital Discovery Assessment (DDA) in the Discover (sales) phase of the SAP Activate Methodology.
The information gathered here should be viewed as a starting point, because it would be difficult for the person completing the DDA to remember every single application that may need to send data to, or receive data from SAP S/4HANA Cloud.
This is another reason why Fit-to-Standard workshops are important; the line of business (LoB) configuration experts will likely uncover at least few additional applications that need to be connected to SAP S/4HANA Cloud while conducting each workshop with the customer LoB experts.
After all Fit-to-Standard workshops have been completed, all integration requirements should be documented in SAP Cloud ALM for the relevant business processes and should now visible in the Requirements app.
The lead integration experts from the partner and customer project teams can then begin analyzing, designing, and documenting their integration strategy with the Integration Solution Advisory Methodology (ISA-M).
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Learn how to navigate each step of the ISA-M in this SAP Blog: SAP Integration Solution Advisory Methodology: Template version 4.0 available now.