Start with behavior, not technology
If users access the service occasionally from a shareable link, a website or web app may be enough. Frequent usage, notifications or device-specific capabilities can make a mobile app more appropriate.
Do not start with two platforms by default
Building both a full website and app from day one doubles testing and maintenance surface. Many products are better served by starting with one channel and adding the second when usage justifies it.
Map the user journey
Identify where users discover the service, the primary action, whether login is needed and what brings them back. Those answers matter more than the technology label.
Before starting a project
Use these questions to structure the idea, then send what you have. You do not need a complete technical specification before contacting SDS.