For a system with more than one particle, we often write the complete state as the product of two kets, one for each particle.
For example, if particle
where A has spin Up and B has spin Down Often, we don’t know which is up and which is down so we write the system as a superposition, which reflects the uncertainty of the system. The expression reflects the probability of each possibility. The particles are entangles
There isn’t a real mathematical description of describing this state, so we just label each one, e.g. up and down.