In private-storage descriptions, zero-knowledge usually means a provider is not given the secrets needed to decrypt a specified set of user content; the exact scope must be stated.
Define the scope before trusting the label
The phrase is often used more broadly in product descriptions than in cryptography. A formal zero-knowledge proof is a different concept from a cloud provider being unable to read stored files. A storage product should explain which content is covered and how recovery works, rather than relying on the label alone.
Ask whether account recovery can recreate access, whether sharing changes key custody, and whether service metadata remains visible. File sizes, connection times, account identifiers and billing information can exist without the service holding content-decryption keys.
A concrete way to assess the claim
Draw three columns: content, keys, and metadata. List who can access each during storage, recovery and sharing. If a product's documentation only answers the first column, the privacy description is incomplete. An empty-looking vault screen is not evidence about the provider's key custody.
For an example of a provider separating content protection from metadata, see Apple's iCloud security overview. Its category-specific descriptions are more informative than assuming a single label covers everything.
How to describe NullVault accurately
NullVault documents local encryption, account-free local use and platform-specific cloud capabilities. Use those precise statements from its security architecture. Do not interpret account-free use as a promise that no operating system, store or network metadata exists. Continue with who can see encrypted photos and local-first privacy.