What is 'index ANDing" exactly?

ANDing means when multiple indexes are used in a single hint, the query optimizer enforce the order of the indexes in the index hints, ANDing applies as many conditions as possible on each index accessed.

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INDEX (index_value [,... n ] ) | INDEX = ( index_value)
The INDEX() syntax specifies the names or IDs of one or more indexes to be used by the query optimizer when it processes the statement. The alternative INDEX = syntax specifies a single index value. Only one index hint per table can be specified.

If a clustered index exists, INDEX(0) forces a clustered index scan and INDEX(1) forces a clustered index scan or seek. If no clustered index exists, INDEX(0) forces a table scan and INDEX(1) is interpreted as an error.

If multiple indexes are used in a single hint list, the duplicates are ignored and the rest of the listed indexes are used to retrieve the rows of the table. The order of the indexes in the index hint is significant. A multiple index hint also enforces index ANDing, and the query optimizer applies as many conditions as possible on each index accessed. If the collection of hinted indexes do not include all columns referenced by the query, a fetch is performed to retrieve the remaining columns after the SQL Server Database Engine retrieves all the indexed columns.