What feature of outbound messages is important considering that they must send both Opportunity and line item data?

The important feature to consider for outbound messages when they need to send both Opportunity and line item data is that they do not support multiple object types. This limitation means that if there is a requirement to transmit information from different object types, such as Opportunities and associated line items, using outbound messages becomes challenging because each outbound message can only pertain to a single object’s data.

In the context of integrating Salesforce with external systems, this restriction implies the need for alternative methods, like using a custom integration that can handle these relationships or using different types of messaging strategies that can encapsulate data pertaining to multiple objects. This emphasis on single object types is crucial for architects and developers to keep in mind while designing robust integration solutions involving Salesforce.

Choosing this option highlights the need to carefully plan how data from related objects (like Opportunities and line items) will be communicated, as traditional outbound messages won't suffice for scenarios requiring combined object data transmissions.