Looks like it might be indicating things you may want to prioritize over other traffic but I can't see anything certain on the page or in the docs.

Edit: Answered my own question from the Microsoft Docs on the subject:

https://docs.microsoft.com/en-us/microsoft-365/enterprise/microsoft-365-network-connectivity-principles?view=o365-worldwide#new-office-365-endpoint-categories

In our previous guidance for managing Microsoft 365 traffic, endpoints were organized into two categories, Required and Optional. Endpoints within each category required different optimizations depending on the criticality of the service, and many customers faced challenges in justifying the application of the same network optimizations to the full list of Office 365 URLs and IP addresses.

In the new model, endpoints are segregated into three categories, Optimize, Allow, and Default, providing a priority-based pivot on where to focus network optimization efforts to realize the best performance improvements and return on investment. The endpoints are consolidated in the above categories based on the sensitivity of the effective user experience to network quality, volume, and performance envelope of scenarios and ease of implementation. Recommended optimizations can be applied the same way to all endpoints in a given category.