Lineage specification refers to the process in which cells in a developing organism decide what type of tissue or cell they will become. It determines their future identity and role in the body. Full definition
In this way embryo - derived stem cell lines and ES cell differentiation may be providing access to potential «transition states,» required for lineage specification in vivo. (journals.plos.org)
ES cells are derived from a stage of development in which key early lineage specification events are occurring. (journals.plos.org)
Reference: Norah M.E. Fogarty et al. «Genome editing of OCT4 reveals distinct mechanisms of lineage specification in human and mouse embryos.» (cam.ac.uk)