Given a record, MultiTableSearch (Regarding; in React, regarding) searches only the rows about it: each table through the column that links its rows to the record — its regarding column. It is the search a record's page has, beside its timeline and its grids. Every row is still read with the visitor's permissions.
The demo's featured customer, as its Account 360 page shows it: its contacts, opportunities, child accounts and notes, grouped by table. See the Multi-Table Search documentation for how a table's link to the record is registered.