What’s the difference between subdomain vs sub-brand isolation?

Subdomain versus sub-brand isolation:

Subdomain isolation:

Different subdomains under same root (marketing.tidalmail.com)

Maintains brand connection

Partial reputation isolation

Easier for recipients to recognize

Sub-brand isolation:

Completely different domains (tidalupdates.com)

Maximum reputation isolation

No inheritance from main brand

Requires separate warmup

Recipients may not recognize relationship

When to use sub-brand:

Need complete isolation (cold outreach)

Main domain is severely damaged

Different business unit with own identity

Sub-brand is stronger isolation but loses brand continuity.

Subsidiary company versus company division. Different levels of separation from parent.