The precious decoration, the sculptures, the water features and the single latrine prove that the building was part of the imperial quarters of the Villa.
Despite the stadium shape, certainly it was not a stadium, just as the so-called stadium of the Palatine palace in Rome was not a stadium.
Instead, it was an inner and secluded garden with a summer triclinium in the center, located under an open and airy pavilion.
It was the private triclinium of the Villa, for the exclusive use of the emperor, while the monumental triclinium of the Canopus was meant for official public receptions.
The Triclinium o the Garden Stadium finds comparisons in the Auditorium of Maecenas in Rome which also had a nymphaeum with steps; it was decorated by trompe l'oeil frescoes reproducing a garden, similar to the one that was seen from the Triclinium NS17. It seems to have been transformed into a triclinium by Tiberius.
Another comparison is the Voliera of Varro, also in Rome, which had the same destination.
See n. 4 - The Imperiale Residence of Villa Adriana in the section Discover the Villa
SEE: Marina De Franceschini, Villa Adriana. Mosaici, pavimenti, edifici. Roma 1991, pp. 216-224 and 502-512.
Neuerburg 1965; Hoffmann 1980; De Franceschini 2006; Sgalambro 2009.