A software developer faces dozens of constraints, from tools he may use, languages his employer supports, hardware and network availability, et cetera. But the largest constraint of software design is what lies between everybody's two ears: the capacity of their brains.
When you are initially designing a system and have finished your requirements solicitations and focus sessions, the first challenge that pops up is how to structure the functionality of the system into something that overlaps adequately to the mental capacities of those who will use it. They possess a certain capability to remember paths of navigation and where to set various field values to achieve specific business objectives.
Brain capacity also tends to limit the creative canvas of developers: you need to know enough of a subset of current technologies to be useful, yet you have to leave adequate "internal storage" to accommodate the intricacies of the domain knowledge for your employer and their industry.
Hence brain capacity planning is similar to bonsai gardening: the constraints force you to be artistic within the limits of your containers.