A soft storage limit means that you can reach and exceed that limit without noticing any negative side effects in your application. However, once you've gone past the soft storage limit, we will contact you and suggest that you consider upgrading to a larger plan. We may also need to move your database to a different server, in order to balance the load more appropriately on our end. Most of our plans use a soft storage limit.
A hard storage limit means that once you have reached the limit, any attempts to write a document into your database will result in an exception. Reads will continue to work without issue, as will indexing. The only plans that use a hard storage limit are the Bronze standard and Bronze replicated plans.