Yechidah, the highest level of the soul, is treated in Jewish mourning custom mainly by its absence from specific practice: because it is held to be the point of the soul's utter union with its divine source, rarely if ever fully manifest in an individual life, the mourning literature offers no distinct rite said to elevate it the way study is said to benefit Neshamah or charity Nefesh. Its place is acknowledged doctrinally rather than addressed through separate custom.
This follows the same historical layering that shapes Chayah's place in mourning practice: the fivefold soul scheme, with Chayah and Yechidah added above the three older levels, Nefesh, Ruach and Neshamah, developed later than the biblical and early rabbinic vocabulary those three levels draw on, and established mourning custom crystallised largely before the higher two were given their fuller later treatment.
In the widely used correspondence scheme Yechidah is placed above the four lettered worlds altogether, or identified with the very highest of them, reflecting its description as the point of union rather than one more distinct level standing alongside the rest, closest of all five to the source Kabbalistic cosmology holds every level of soul to ultimately share.