For example you could lookup a user's profile based upon their DID, their Ethereum account, or their Twitter handle; or you could lookup a user's Filecoin account based on their Polkadot account. We are dealing with The Graph on indexing these identity structures which will open up many new chances in this realm.
If you likewise need a method of storing and handling user information, you have three primary choices. Use https://squareblogs.net/lowdamage4/things-about-facebookincubator-idx-library-for-accessing-arbitrarily-nested to directly save documents on the Ceramic Network and include references to those documents in the user's index. For this choice, no extra software application is needed; it works out of package with IDX.Store data in alternative data storage systems such as Filecoin, IPFS, Sia, Arweave, Textile, Orbit, DB, Secure Data Stores, or Ethereum agreements and use IDX to include referrals to this data in your user's index.
This is real regardless of where the data resides (servers or decentralized networks) or which application first developed the information. The identity index consists of mappings to various information sources, Share information across applications and silos, As explained above, the essential element of IDX that de-silos details, promotes interoperability, and makes it possible for user control is the identity index.