So far what I had understood about potential energy is that it is defined for a System(With altleast two particles) with forces acting towards each other of same magnitude and ,
$$-W_\mathrm{conservative} = ΔU$$

For example in case of gravitational potential energy ,we only consider work done on small body as there is negligible work done on the earth. 

But in case such as : The left side of a massless spring is attached to the wall, and there is a block attached to the right side of the spring. 

If we consider block + spring as our system then the the net work done by internal forces would be zero , So what is increasing the potential energy of the system then ?

I refer to this question - [Question][1]

According to some answer , I thought I need to consider the earth also into system and Spring just stores potential energy like gravitational field not really being the part of the system.

Is this reasoning correct ? If not please help me clear these doubts.

Thanks in Advance ;)


  [1]: https://physics.stackexchange.com/questions/553741/potential-energy-of-a-spring-vs-potential-energy-of-the-spring-mass-system