Question on In-Row varbinary(max) on SQL FILESTREAM

I have a question on what actually gets stored in a data row for a varbinary(max) column setup for SQL FILESTREAM. (with FILESTREAM attribute)

Do we have to take into account on the dynamic size of that particular blob within a data row?

In other words, is it recommended to store our large value types out of row for a varbinary(max) FILESTREAM use-case? Or does it only store a fixed # of bytes on that specific column (GUID or whatnot) which references the local file location of the blob?

sp_tableoption N'MyTable', 'large value types out of row', 'ON'

Thanks.

Samuel