From Halopedia, the Halo wiki
Harborages were types of gargantuan space station and starship employed throughout the Covenant empire. With the empire's devolved leadership structure allowing individual worlds considerable autonomy, the Hierarchs used harborages to control the stars and the space between planet - ensuring a complete and total control over their imperial subjects. Harborages were spread throughout the empire and served as regional capital-sites, consisting of various classes of city-ships and command-and-control centres which were able to broadcast the will of High Charity. Alongside this function, they served as replenishment yards for the Covenant fleet, safeguards for Forerunner artefacts and shipyards for the military. Harborages may have some broad overlap with the Covenant's starship classification of Exarchs.
The importance of harborages was absolute, and as such even the hierarchs were unable to move them from their appointments unless absolutely necessary, due to the risk of unveiling their carefully hidden secrets. Two such installations were encountered by humanity during the Human-Covenant War; the Ring of Mighty Abundance at Zhoist and Unyielding Hierophant in the Tau Ceti system. Both were destroyed via the actions of the SPARTAN-IIs, acting as dark omens for humanity's eventual victory in the war.
The largest classes of supercarrier, such as the Sh'wada-pattern, operated on a similar size scale to harborages like Unyielding Hierophant and were said to have more in common with city-fortresses than conventional warships - serving many similar functions to known harborage examples. As such, these massive vessels may also be considered harborages.
List of appearances