PII is a special tag in Select Star, where the value will be removed in-memory from SQL queries where they are exposed.
To be sure you are tagging all downstream sources, click on the database in the left sidebar. Change the Data Type filter to Columns, and use Cmd/Ctrl+F to filter the results to find the column you're looking for.
Select the column and click Edit Tags to open a modal. Here you'll be able to to see every place this column is used, and be sure you are masking the data in each place it appears.
You can determine if PII values will can be exposed in SQL queries if they are used As Is. This means it is possible to write a query which shows PII directly, such as SELECT * FROM customers c WHERE c.ssn = '555 66 7777'.
Click the arrows to show downstream sources. You can select the PII column in as many places as it appears and apply the PII tag.
Any queries using data with the PII tag will be masked as shown.
If you need to see the PII data or have tagged something as PII by mistake, remove the tag and the queries will no longer be masked the next time your metadata sync runs.