What does "access to all assets" mean for Business Unit members vs. Default Asset Permissions?

In the Eloqua Help Center documentation on Business Unit membership, it states: "Users who are part of a business unit have access to all of the assets and data within those business units."

I'd like to understand exactly what "access to all assets" means in this context.

My understanding is that in the Default Asset Permissions settings, you can specify which security groups have visibility into an asset at the time it is created. I'm not sure how business unit membership-based access differs from this security group-based permission model, and I'd appreciate any clarification.