Good question Mike. For sure all trees grow and that generates elongation of branches. The term elongating species is not some formal botanical term. it is a term used by Ryan Neil from Bonsai Mirai to categorize essentially all species of evergreen conifers that are not either pines or junipers. This includes Taxus, redwoods or Larix (both deciduous conifers), Thuja, Chaemacyperis, Tsuga, Pseudotsuga, Picea, etc... based on the behaviour of their growth and response to pruning that is totally different from pines and junipers or broadleaf evergreens (like boxwood) or broadleaf deciduous. Elongating species you can pinch in the spring to stop elongation (and eventual hardening of that growth) and promote the growth of already existing internal growth or the development of buds in the interior (where depends on the nuances of the species).