Muʿasarah (contemporaneity): two narrators lived during overlapping years; this alone does not prove that they met.
Liqaʾ (meeting): the individuals are shown to have met; that does not itself prove that a particular report was heard at that meeting.
Samaʿ (hearing): a narrator actually received the report from the named teacher, or its transmission is established through an applicable valid mode.
For example, shared dates of life show the possibility of contemporaneity; a documented meeting is stronger evidence; explicit hearing of a particular report is more specific still.
Confusing these layers can lead to mistakes in evaluating a muʿanʿan chain.