How does combinations in the matrix table gets created

Summary:

I would like to know the logic to create combinations in the matrix table and which query can i use to check tables data.

Content (please ensure you mask any confidential information):

Sometimes it happens that I load data at an aggregate level to a custom measure for two items and data is visible in the Demand and S&OP tables for one item but not for the other, and I didn't load data at lowest level, so I assume that there is a combination for the data that I can see in the plans.

According to documentation, if I load data to a custom measure at an aggregate level it won't be visible in Demand Plan or S&OP plans because combinations are not created in the matrix table (see