If one dimension is very large compared to the others, the principal strain in the direction of the longest dimension is constrained and can be assumed as constant, that means there will be effectively zero strain along it, hence yielding a plane strain condition (Figure 7.2).
If one dimension is significantly larger than the others, the principal stress in that direction will be constrained and can be assumed constant, resulting in effectively zero strain along it, hence yielding a plane strain condition (Figure 7.2).