As stated in DIS: “Mirrors”, an erigah is a Breen blood bounty.
The ship holding Moll and L’ak’s shuttle in a tractor beam is the USS Locherer (NCC-325062), a Merian-class starship first seen in DIS: “Jinaal” and named after the late J.P. Locherer, who was a cinematographer on the show.
Burnham identifies subspace frequency Epsilon 19 as a courier channel and that “special offer” is a courier distress code. With her is Commander Nhan, a Barzan who journeyed with Discovery from the 23rd Century but subsequently transferred to Federation Security in the 32nd. She was last seen in DIS: “Rubicon”.
Nhan refers to what happened between her and Book at their last encounter, when she argued for destroying his ship to prevent him using the isolytic weapon she mentioned. So there’s a bit of awkward history there.
A sa-te kru cat is a Vulcan species, a large predator similar to the le-matya. It was mentioned in the novel Vulcan’s Forge, but this is its first on-screen mention.
Given the Breen first appeared in DS9, there are plenty of back references to the series.
Culber says that there’s some evidence Breen are capable of “somatic cell” regeneration in extreme cold. This tracks with reports that the Breen homeworld had a freezing climate (DS9: “Til Death Do Us Part”) and that they wore refrigeration suits (DS9: “The Changing Face of Evil”). I’m not sure why Culber needs to distinguish “somatic cell”, since that is really any other cell in the body aside from sperm and egg cells, but I guess it sounds medically cool.
Breen Dreadnoughts (Rezeth Destroyers) are ships from Star Trek Online. As we see later, the 32nd Century version is much bigger.
The Breen used to be a Confederacy in the 24th Century (DS9: “Strange Bedfellows”) but somewhere along the way it’s become an Imperium. There are six primarchs vying for the throne in the wake of the emperor’s death.
Rayner talks about the last time the Breen entered Federation space. With Vance saying that Starfleet was caught flat-footed, this is probably referring to the Breen sneak attack on Earth during the Dominion War which heavily damaged Starfleet Headquarters and San Francisco (“The Changing Face of Evil”). Tilly’s later remark about the Breen “destroying an entire city” may also refer to this.
Using thoron emitters and duranium shadows to fool enemy sensors is a reference to DS9: “Emissary”, when the station used such a tactic to block sensors and make themselves appear better armed than they were. In DS9: “The Way of the Warrior”, the Changeling Martok believed the station was pulling the same trick, but that time he proved to be wrong. The Romulan saying “Never turn your back on a Breen” is from DS9: “By Inferno’s Light”.
The yellow alert symbols are the same design as the “Alert: Condition Red” indicators dating back to ST II, albeit in yellow. The USS Mitchell (NCC-325027), another Merian-class starship, is named after the late Kenneth Mitchell, who played Kol, Kol-sha and Aurellio in DIS. She was last mentioned in DIS: “Coming Home”.
L’ak is Primarch Ruhn’s nephew and carries within him the genetic code of the Yod-Thot, “they who rule”. He is also a direct descendant of the emperor and Ruhn cannot claim the throne without him.
One of Reno’s former jobs was as a bartender - the closed captioning says “Ashalon IV”, but it might be a misspelling of “ Aschelan IV”. Aschelan V was a planet which housed a Cardassian fuel depot (DS9: “Dreadnought”). She refers to a cocktail named “Seven of Limes”, which is an obvious pun on Seven of Nine, although Reno may not know the name’s provenance given that she left for the future about a century before Annika Hansen was assimilated.
A “Code One Alpha” is probably related to or the same as the 23rd-24th Century “Code One Alpha Zero” which is an emergency condition ordered when there is an attack (ST 2009) or a distress call (TNG: “Relics”).
Kellerun was, for a time, used as a Breen forward base by Primarch Tahal. Since Rayner was there, it must have been relatively recent, although to be fair we don’t know how long Kellerun live.
Reno remarks that the hunt “sounds like something out of a holodeck adventure for the littles.” She may be referring to The Littles, a series of children’s adventure novels featuring a family of tiny humanoids with mice-like faces and tails that were written between 1967 and 2003 by John Peterson. There was also an animated series that ran for 3 seasons between 1983 and 1985. Or I’m overthinking and she’s just talking about kids in general.
Rayner says Tahal named her ship the Tau Ceti after a lethal viper with a slow acting venom. Tau Ceti, is of course, the name of a star 12 light years away from Earth and has been mentioned many times in Star Trek.
Bopak III was an uninhabited planet (at least in 2372) in the Gamma Quadrant and the location for the events of DS9: “Hippocratic Oath”.
Tricordrazine is a stimulant apparently derived from cordrazine (TOS: “The City on the Edge of Forever”) and appeared in several TNG episodes, including TNG: “Ethics” and “Shades of Grey”.
The Badlands is an area of space in proximity to Cardassia and Bajor (and DS9), known for its violent plasma storms (DS9: “The Maquis”). Both the Bajoran Resistance and the Maquis used it as a staging area to hide from enemy patrols during their respective conflicts. In 2371, while pursuing a Maquis ship there, the USS Voyager was hurled across the galaxy to the Delta Quadrant by an alien force (VOY: “Caretaker”).