The tripartite meanings of being, from Lee Braver's A Thing of This World
But we have one more important step to take in order to understand Heidegger’s later work—the step from the epochal understandings of Being (or beingness) to Being itself, also called the event of appropriation (Ereignis), the truth of Being, Beyng, or the clearing at various times. Being itself is the source (though not the cause, which would make it a being) of all the epochs, the “sender” of the sendings. As usual, language gets in the way, requiring many caveats and qualifications: “One can name it an origin, assuming that all ontic-causal overtones are excluded: it is the event [Ereignis] of being as condition for the arrival of beings: being lets beings presence” (P. 59; see also pp. 414–5). This tripartite structure made up of (1) beings, (2) beingness or the understanding of Being, and (3) Being itself is, I believe, the central organizing point of all of Heidegger’s later thought, yet it has given rise to a great deal of confusion, much of which could have been averted had he employed a clarified terminology.[Sheehan, Thomson] He himself admits to having created some confusion due to the ambiguity of the word “Being,” which can easily apply to both (2) beingness and (3) Being itself [The German word for “a being,” das Seiende, is not the same as the word for “Being,” Sein.] (Pp. 20, 26–27; Martin Heidegger in Conversation p. 44).Continued
Heidegger casts about for a new way of writing to express these difficult ideas—crossing out the word “Being,” employing terms like Ereignis, “the fourfold,” Spielraum, “the It gives” (es gibt), even coming to reject the ontological difference itself, since it focuses on the difference between beings and beingness: “From the perspective of Appropriation it becomes necessary to free thinking from the ontological difference” (P. 37; see also p. 300). There are times, though, when he speaks of the tripartite structure fairly clearly.
We can say, in summary, that three meanings can be emphasized in “letting-be.” The first refers to  that which is (to the being). Over against this first sense, there stands another sense for which the attention is drawn less towards what is given (towards what is), than towards  the presencing itself. It then concerns an interpretation of being of the sort given by metaphysics. Within this second emphasis, however, a third has its place, where the stress is now decisively placed upon  the letting itself, that which allows the presencing. . . . In this third meaning, one stands before  being as being, and no longer before [A3] one of the  forms of its destiny. If the emphasis is: to let presencing, there is no longer room for the very name of being. Letting is then the pure giving, which itself refers to the it [das Es] that gives, which is understood as Ereignis. (Pp. 59–60, bracketed comments added; see also p. 62; P. 19)There are (1) ontic beings, which are just the entities we encounter and deal with day to day. According to ICS, these entities are determined by (2) ontological beingness, that is, an overall defining character which varies from epoch to epoch and which is captured best in the writings of metaphysicians. Finally, there is (3) the very emergence or giving or unconcealment (or truth) of these epochal modes. “In the beingness of beings, metaphysics thinks  being, yet without being able to ponder the  truth of being” (P. 232, bracketed comments added; see also pp. 322, 375). Although most of Heidegger’s analyses of the history of philosophy remain at the second level of understandings of Being, his ultimate goal is to point us toward this third level. Ordinary people focus on beings, their experience being guided by an unthematic (pre-ontological) understanding of these entities’ epochal beingness. Metaphysicians have always transcended beings to focus explicitly on beingness, but philosophers since the pre-Socratics have forgotten or ignored the third level—Being itself or Ereignis (P. 444). “In the beginning of Western thinking,  Being is thought, but not  the ‘It gives’ as such. The latter withdraws in favor of  the gift which It gives. That  gift is thought and conceptualized from then on exclusively with regard to  beings.”[P. 8, see also p. 51; p. 443; pp. 189–90]