Full Idea
The two most fundamental modes of being in Sartre's ontology are being in-itself, and being for-itself. ...The in-itself lies beyond our experience of it.
Gist of Idea
For Sartre there is only being for-itself, or being in-itself (which is beyond experience)
Source
report of Jean-Paul Sartre (Being and Nothingness [1943]) by Christine Daigle - Jean-Paul Sartre 2.2
Book Reference
Daigle,Christine: 'Jean-Paul Sartre' [Routledge 2010], p.32
A Reaction
This appears to be Kant's ding-an-sich, paired with Heidegger's Dasein. If those are the only options, then reality is either subjective or unknown, which seems to make Sartre an idealist, but he asserted that phenomena vindicate the in-itself.