Atlan propagates tags based on hierarchy and lineage, which means tags can be propagated to connected assets. For example, if you tag a table as PII and that table has downstream columns, Atlan will tag those downstream columns as PII as well — only if propagation is enabled.
Atlan does not auto-detect PII data. Atlan will only propagate the PII tag to downstream assets if you have enabled tag propagation manually or automated the task using playbooks.