In Power Query you can choose detailed data types for columns e.g. Int8.Type. When I do this, it shows the Number symbol (1.2) above that column rather than the Integer symbol.
Does using Int8.Type reduce the memory used to store a column in Power BI?
I haven’t investigated M specifically, but in SQL Server the number relates to how much memory space is allocated, so a best practice was to always use the smallest size that would handle your data and the transformations you plan. Given both are Microsoft, I think it’s not unlikely the same situation applies to both.
Greg