What is a limitation when using variables in aggregates?

The limitation regarding the use of variables in aggregates is that they cannot be used directly within the context of aggregations. This means that while you can create and manipulate variables in your logic, you cannot use those variables within aggregate functions themselves. Instead, you must use input parameters or other means to pass data into aggregated computations.

Aggregates are typically designed to work with data sources directly, such as database entities or lists, to perform operations like SUM, COUNT, or AVERAGE. By not being able to use variables directly, it maintains a clear distinction between raw data handling and the variables meant for operational logic, encouraging a structured approach in the programming model.

The other options present misconceptions about variables. Variables are not limited to being defined at a global level; they can be scoped locally as needed. Additionally, variables can hold various data types, including but not limited to complex types like structures and lists, not just singular types like integers. This flexibility allows developers to craft more complex and capable applications.