What deals with identifying the various functionalities, data, inputs, outputs, deliverables, artifacts, etc. of the intended solution.
These are examples of the kinds of questions you should be asking with respect to the “What” of the intended solution:
What are the goals and objectives of the solution (should be stated in business terms)?
What do the users need to be able to do in/with the solution?
What data elements does the solution need to capture, store, generate, output, etc.?
What are the master data elements?
What are the transactional data elements?
What is the authoritative source for the master data elements?
What business rules does the solution need to enforce?
What processes does the solution need to support and/or facilitate?
What transactions does the solution need to suport and/or facilitate?
What other systems does the solution need to interact with?
What data elements does the solution need to exchange with other systems?
What are the volumetrics for the solution (e.g., # of users, # of transactions, data growth rate, etc)?
What problems does the solution solve or mitigate?
What am I missing?