The chemical characterisation of halo substructure in the Milky Way
  based on APOGEE
  Galactic haloes in a $\Lambda$-Cold Dark Matter ($\Lambda$CDM) universe are
predicted to host today a swarm of debris resulting from cannibalised dwarf
galaxies that have been accreted via the process of hierarchical mass assembly.
The chemo-dynamical information recorded in the Galactic stellar populations
associated with such systems helps elucidate their nature, placing constraints
on the mass assembly history of the Milky Way. Using data from the APOGEE and
\textit{Gaia} surveys, we examine APOGEE targets belonging to the following
substructures in the stellar halo: Heracles, \textit{Gaia}-Enceladus/Sausage
(GES), Sagittarius dSph, the Helmi stream, Sequoia, Thamnos, Aleph, LMS-1,
Arjuna, I'itoi, Nyx, Icarus, and Pontus. We examine the distributions of all
substructures in chemical space, considering the abundances of elements
sampling various nucleosynthetic pathways. Our main findings include: {\it i)}
the chemical properties of GES, Heracles, the Helmi stream, Sequoia, Thamnos,
LMS-1, Arjuna, and I'itoi match qualitatively those of dwarf satellites of the
Milky Way, such as the Sagittarius dSph; {\it ii)} the abundance pattern of the
recently discovered inner Galaxy substructure Heracles differs statistically
from that of populations formed {\it in situ}. Heracles also differs chemically
from all other substructures; {\it iii)} the abundance patterns of Sequoia
(selected in various ways), Arjuna, LMS-1, and I'itoi are indistinguishable
from that of GES, indicating a possible common origin; {\it iv)} the abundance
patterns of the Helmi stream and Thamnos substructures are different from all
other halo substructures; {\it v)} the chemical properties of Nyx and Aleph are
very similar to those of disc stars, implying that these substructures likely
have an \textit{in situ} origin.
