Explain implicit sharing and why it matters to an architect.

Implicit sharing is access the platform generates itself, without any rule you configured.

1. Parent implicit sharing: A user who gains access to a Case, Contact, or Opportunity automatically receives read access to the parent Account. It is read-only and it covers only the parent, not the parent's other children.
2. Child implicit sharing: The Account owner may gain access to child Contacts, Opportunities, and Cases owned by others — but only to the level set by the Contact Access, Opportunity Access, and Case Access settings on their role.

It matters for two reasons. First, it is the explanation for a whole category of confusing tickets — an account owner who cannot see opportunities on their own account almost always has a role-level access setting at No Access, and no amount of Opportunity sharing rule tuning will fix it.

Second, implicit share rows are generated in volume. In a large org they can dominate the sharing tables, which drives recalculation duration when you change an OWD or reorganise roles.