Roll-up summary fields have specific characteristics. They must be created on the master object in a master-detail relationship to aggregate data from detail records (B). Furthermore, the existence of a roll-up summary field on the master object creates a dependency that prevents the master-detail relationship from being converted to a lookup relationship until the field is deleted (C). These are fundamental considerations for data modeling in Salesforce.

A. The available roll-up types are COUNT, SUM, MIN, and MAX. AVG (Average) is not a standard function for roll-up summary fields.

D. Roll-up summary fields are calculated and updated asynchronously after a record on the detail side of the relationship is saved, deleted, or undeleted.

- Salesforce Help, "Roll-Up Summary Field". This document explicitly states, "You can create roll-up summary fields on any master object in a master-detail relationship." It also lists the available functions as COUNT, SUM, MIN, and MAX.

- Salesforce Help, "Considerations for Relationships". Under the section for converting relationships, it notes, "You can’t convert a master-detail relationship to a lookup relationship if the master object has a roll-up summary field that summarizes a field on the detail object."

- Salesforce Object Model and Customization, Trailhead Module "Data Modeling". Unit on "Create Object Relationships" covers the properties and limitations of roll-up summary fields.