Within the TOGAF specification, you will sometimes read about there being two options for approaching the design of architecture: a top-down approach or a bottom-up approach.
But what does that mean?
The top-down approach means that you start with the business requirements (the B layer) and fully decide the problems your business needs to solve before thinking about the technology to solve those problems. The “bottom up” approach means you start with the technology that you are looking to add to your organization and work “up” to change the applications and the business.
An example of this is, let’s say you have a problem with security. You talk to the business leaders, department heads, etc and come up with an extensive target business architecture for it. Then you start looking at vendors and thinking about implementation. That’s top-down.
The bottom-up approach is, when you have a problem with security, talking to the technical teams and ops teams. What can you do to improve security? They might suggest you restrict all access from outside your company except through the VPN. You then work up to the business layer to say that certain policies need to be followed to get access to the company network from outside. That’s a top up approach. Thinking about solutions first and then changing the business needs to match.
Hope that helps.