Schema Designer goes further than the custom-field builders elsewhere in the product (portal category fields, CI class attributes) — it adds genuine, permanent fields and tables to the underlying platform, not a flexible catch-all bucket of extra data. That makes it more powerful and more permanent than everyday customization, which is why it's only available on certain deployments.
Adding a field to a core record type
Pick a target — Incident, Change, Problem, Asset, or a specific CMDB CI class — and add a field: a label, a type (text, number, date, yes/no, a dropdown of choices, or a reference to another record), whether it's required, a default value, and where it appears relative to other fields. A field can be deactivated later without losing its data, or permanently dropped — dropping requires typing the field's exact name to confirm.
Building an entirely new table
Beyond adding fields to existing record types, Custom Tables lets you define a brand-new record type from scratch — give it a name and description, add its initial fields, and it gets its own basic create/view/edit/delete screen. Dropping a custom table is permanent and, like dropping a field, requires typing its name to confirm.
Example
A logistics company needs to track shipment tracking numbers on Changes, something no existing field covers. An admin adds a "Tracking Number" text field to Change through Schema Designer — it appears on every change form going forward, stores real data like any built-in field, and can be reported on the same way.