“Custom software” is a broad term that can include desktop applications, web apps, websites, and systems made up of many of these. Mobile apps can be created using an off-the-shelf mobile app builder, they can be custom developed from the ground up, or they can be built using a strategy in between.
Some mobile apps (like games) are standalone systems—everything is managed within the mobile app.
Most mobile apps, however, are one element of a larger system. Think about your bank. They probably have a web portal for online banking as well as a mobile app. You can accomplish similar tasks in both, and the data is the same.
When you’re building a new custom software system, it’s important to think about it in terms of your overall software strategy. Do you need a desktop application for your internal team to use, which connects to a web application for customers to use? Do you need mobile apps for customers, too? Think about where users will be using the system, what they’ll be trying to accomplish, and how you can create the best experience.