# Why SdfSchemaBase and SdfSchema?
**URL:** https://forum.aousd.org/t/why-sdfschemabase-and-sdfschema/3023
**Category:** USD
**Tags:** cpp
**Created:** [August 24, 2026, 8:28pm UTC](https://forum.aousd.org/t/why-sdfschemabase-and-sdfschema/3023 "2026-08-24T20:28:14Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@maddy-adams](https://forum.aousd.org/u/maddy-adams)
#### Post date: [August 24, 2026, 8:28pm UTC](https://forum.aousd.org/t/why-sdfschemabase-and-sdfschema/3023/1 "2026-08-24T20:28:14Z")
Why are `SdfSchemaBase` and `SdfSchema` different classes? Is this just a historical artifact, or is there a reason they can’t be merged/treated as the same?
- Are there other classes in OpenUSD that derive from `SdfSchemaBase`?
- Are users expected to sometimes write custom classes that derive from `SdfSchemaBase`, e.g. while writing an `SdfFileFormat` plugin?
- Could there be multiple distinct `SdfSchemaBase` instances during program execution which aren’t the immortal `SdfSchema` instance?
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### Author:  [@spiff](https://forum.aousd.org/u/spiff)
#### Post date: [August 25, 2026, 12:57am UTC](https://forum.aousd.org/t/why-sdfschemabase-and-sdfschema/3023/2 "2026-08-25T00:57:28Z")
> [@](#):
>
> - Are there other classes in OpenUSD that derive from `SdfSchemaBase`?
There are not. It is required for Pixar because Presto builds atop OpenUSD with its own “Sd” document model, in which all Presto/Sd layers **are** SdfLayers, but which have a SdSchema that is distinct from SdfSchema.
> [@](#):
>
> - Are users expected to sometimes write custom classes that derive from `SdfSchemaBase`, e.g. while writing an `SdfFileFormat` plugin?
We don’t expect anyone to have a need to do that, and it would create interoperability issues that we deal with in Presto only because we need to. In a future world we hope to eliminate our own need, but it’s going to be awhile.
> [@](#):
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> - Could there be multiple distinct `SdfSchemaBase` instances during program execution which aren’t the immortal `SdfSchema` instance?
That is the case in Presto. You will also soon be seeing some of the “usdMode” checks in Pcp being replaced by checks of a layer’s schema being the SdfSchema or not, as we begin the work that will ultimately allow us to eliminate multiple Schemas.