Because most of developers have 2008 ssrs
FYI 2008 and 2008 R2 create slightly different RDL.
My question is : I remember visual studio 2010 does not have SSRS projects model, it can only edit local c# , no SSRS reports. So how this 2010 comes from?
BIDS that comes on the SQL 2008 R2 CD is built on VS 2010. The developer likely opened it in this.
secondly, that report definition is 2010, but the server is SSRS 2008 R2, so does that mean 2010 report works on 2008R2?
BIDS 2008 -> Built on VS 2008
BIDS 2008 R2 -> Built on VS 2010
How can I open this report in SSR2 2008 to edit it?
I saw an article once that you can convert the XML from 2008 R2 back to 2008 manually, by editing the XML file by hand to remove R2 references. It might be easier to just recreate the report.