Dear all
When some flange or any rigid body is attached within two pipe diameter after welding point of a elbow we pick pipe bend type “Single flange” & if both the side of elbow have rigid body we pick “Double flange” as mention in caesar II manual.
I have seen some people use Single or double flange concept even if elbow have fitting to fitting connection with other piping item like reducer, tee.
Is there any clear cut direction or authentic document on the same?

I have not seen any documentation in a Code on this.

You need to think about exactly what you are representing in the Caesar II model. The flexibility in a bend above that of straight pipe is due to the way the bend shell deforms under bending loads. Consider in-plane bending ... That will cause the bend cross section to ovalise. In doing so it reduces the effective section modulus and promotes additional flexibility. But, it also causes local stresses that will ultimately cause a crack if repeated enough times with suffucient amplitude. Tee ovalising effect soes not stop at the bend weld, and is often said to proceed about 3D along the adjacent pipe.

Now. Think about a flange attached to a bend. it will of course stiffen the cross section and reduce available ovalisation, also redicing the sif. The B31.3 code reflects this. So consider your tee now. Is it stiff enough to arrest that ovalisation ? Possibly I would say, due to the thickening of some fittings. A reducer ... probably not, at least if the large end is on the elbow. Personally I don't think most pipe fitting would have the stiffening effect that a flange has, but FE/pipe type analysis would quantify.