Explanation/Reference:
Explanation:
The ADR root directory is known as ADR base. Its location is set by the DIAGNOSTIC_DEST initialization parameter. If this parameter is omitted or left null, the database sets DIAGNOSTIC_DEST upon startup as follows:
If environment variable ORACLE_BASE is set, DIAGNOSTIC_DEST is set to the directory designated

by ORACLE_BASE.
If environment variable ORACLE_BASE is not set, DIAGNOSTIC_DEST is set to ORACLE_HOME/log

Within ADR base, there can be multiple ADR homes, where each ADR home is the root directory for all diagnostic data-traces, dumps, the alert log, and so on-for a particular instance of a particular Oracle product or component. For example, in an Oracle Real Application Clusters environment with Oracle ASM, each database instance, Oracle ASM instance, and listener has an ADR home.