Protocols, interfaces and formats that are published, implementable by more than one vendor, and not dependent on a single supplier staying in business or keeping its prices. This is how the platform is built, not a marketing position.
Four areas where an open standard changes what you can do the following year.
Portability
Identity, storage interfaces and deployment definitions are chosen from published standards, so a workload can be moved without rewriting it. Where a proprietary service is genuinely the better tool, the dependency is written down as a dependency, with the exit path next to it.
Portability is a design decision recorded at build time, not a promise made later.
Mail uses SMTP, IMAP and JMAP; files use WebDAV and standard sharing; identity uses OIDC. Nothing requires a client that only one vendor ships, so your own tools keep working when the platform changes.
If a service can only be reached through one vendor's client, it is not standards-based.
Formats are documented and exports are tested, not assumed. A restore is exercised on the same schedule as a backup, because an archive in a format nobody else can read is not an archive.
Ownership is proven by a successful restore, not by a clause in a contract.
Rules for access, networking and cost are held in version control with the same review process as application code. An audit can show which rule produced which result, and who approved it.
Governance that cannot be read is governance that cannot be checked.
A standard is not automatically the right tool. Managed services win when the operational cost of running something yourself is higher than the cost of the dependency. Those cases are named in the consolidation plan rather than resolved by ideology.