Microsoft Documentation, Data Analysis Expressions (DAX) Reference:
CALCULATE function: The documentation states, "Evaluates an expression in a modified filter context." This is the primary function needed to change how the [Deliveries] measure is calculated.
Source: Microsoft Docs, "CALCULATE function (DAX) - DAX".
USERELATIONSHIP function: The official documentation specifies, "USERELATIONSHIP specifies the relationship to be used in a specific calculation... It is used within functions that take a filter as an argument, such as the CALCULATE... functions." This confirms its role as the modifier inside CALCULATE to activate the inactive path.
Source: Microsoft Docs, "USERELATIONSHIP function (DAX) - DAX".
Russo, M., & Ferrari, A. (2020). Definitive Guide to DAX: Business intelligence for Microsoft Power BI, SQL Server Analysis Services, and Excel (2nd ed.). Microsoft Press.
Chapter 5, "Working with multiple tables": This chapter details the concept of active and inactive relationships. It explains that only one relationship between two tables can be active at a time and that USERELATIONSHIP must be used within a CALCULATE function to enable calculations over an inactive relationship. (Specific patterns are discussed in the "Multiple relationships between tables" section).