This is one of the most common points of confusion in a Halo implementation, and it causes real problems if it isn't resolved early.
The Service Catalogue is your CMDB — it holds the five-layer SDM hierarchy and drives all automation. The Request Catalogue is what your end users see in the self-service portal. They aren't the same thing, and conflating them leads to either a portal full of internal IT plumbing that confuses users, or a CMDB built around what looks good in the portal rather than what actually drives routing.
The key mechanism to understand is the twin: when you create a Technical Service in the CMDB, Halo automatically creates a corresponding catalogue item in the portal, hidden from end users by default. You choose when and to whom each item is visible. This means your SDM build directly populates your request portal — no duplication of effort, no second system to maintain.
The discipline is keeping each catalogue doing its proper job.
In practice, this means agreeing up front — before you start modelling — which services should be visible to end users, and which exist purely to drive internal routing and SLA management. That decision shapes everything downstream. Getting it wrong early creates a portal that confuses users and a CMDB that's been bent to fit the wrong purpose.