ATLAS is separated into two groups, ATLAS-A and -T. The former contains 5584 spectra covering spectral types from O to M-type and some special types such as A supergiant, blue horizontal-branch (BHB), and Carbon stars. All the spectra have absolutely calibrated  fluxes at certain characteristic wavelengths corresponding to the optical passbands with accuracy better than 2.6% by comparing with PanSTARRS1 g, r, and i bands photometry. ATLAS-A contains 5342 directly observed stellar spectra with Teff , logg and [M/H] and 242 without [M/H]. The parameter coverage of ATLAS-A is from logTeff  ∼ 3.5 to 4.7, from MG ∼ −9.5 to 12mag, from [M/H]∼−1.5 to 0.5dex, and from logg ∼ 0 to 5dex. ATLAS-T, which are averaged over fluxes of the directly observed spectra with similar parameters, contains 997 stellar spectral templates with Teff  logg and [M/H] and 121 without [M/H].