What a viewing key is

Every shielded account has a spending key, which moves funds, and viewing keys derived from it, which can only read.

A viewing key lets someone decrypt that account's transactions: amounts, timing and memos. It can't sign anything, so it can't spend. That makes selective disclosure possible: you can show your history to an accountant, auditor or tax authority while it stays private from everyone else.

There are two common kinds. An incoming viewing key reveals payments received. A full viewing key also reveals what was spent. Wallets that use unified addresses export unified viewing keys (they start with uview or uivk).

Sharing is one-way: you can't revoke a viewing key, and whoever has it can keep watching that account's future activity. Share it only with people you trust, and consider a separate account for activity you expect to disclose.