Your conclusions are correct. Regarding your doubts, here are the responses
1. Seismic weight for static seismic analysis won't be calculated by itself and have to be separately defined?
No. When you define a mass type reference load, you DO NOT need to define seismic weight for an equivalent static seismic analysis either. The software would use the Y direction loads out of the mass type reference load to automatically calculate the seismic weight.
2. If I define a reference load with Type 'Dead' or any other type except 'Mass'. Will it be included in any calculation unless otherwise specified?
Reference load types other than Mass are NOT used automatically for other calculations UNLESS there are rigid diaphragms defined ( explained later). These reference load types are used only if these are referenced explicitly in other load cases.
When rigid diaphragms are defined, there is a provision that if there are no mass type reference loads in the model and an equivalent static or dynamic analysis is being done without any seismic weight/mass being defined as part of these load cases, the software would search for Gravity type reference loads and if not present, will search for Dead and Live type reference load cases and use those to compute the mass. Notes will be provided in the analysis output file to make the user aware of that.
3. What happens when in type 'Mass' only Y direction is included. That means where STAAD will use it by itself.
The mass will only be considered in the Y direction which may be ok if you are doing a static equivalent analysis but not if you are doing a dynamic analysis like response spectrum or time history or even just an eigensolution. For dynamic analysis you have to consider the masses in all 3 directions.
4. What happens when Three 'Mass' reference load definition is defined, each for X, Y and Z. Will it function similar as defining Mass in all three directions in just one definition.
Yes