determine grain by the physical realities of the data's source

hello
what does this sentence means in kimball book?
"The grain is determined by the physical realities of the data's source"
but I think grain determine by a business process not the data's source.

how do you think?

I think you should re-read Kimball's book to understand the grain concept and how to define it. Typically I store the data at the lowest grain regardless of the business process. This allow the most flexibility for reporting.

OK, thanks
can you explain "The grain is determined by the physical realities of the data's source",
what does it mean?

Sure, the data defines the grain. There is uniqueness in the data based on the columns. When building fact tables, store the lowest uniqueness since this always leaves the ability to aggregate later. Read the chapter on the grocery store. Data is stored at the product level but the business process may want reporting at the ticket level.