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tokens: The LIGO Scientific and Virgo Collaborations have so far reported 15 confirmed gravitational-wave (GW) signals from the mergers of compact objects (Abbott et al. 2019a , 2020b , 2020c , 2020d , 2020e ). Depending on the nature of the objects and the environment surrounding the binary, merging compact binaries can also emit electromagnetic radiation and neutrinos. In the second observing run (O2) of Advanced LIGO and Advanced Virgo, low-latency GW candidates were distributed to partner astronomers for multi-messenger follow-up (Abbott et al. 2019b ). This partnership resulted in the combined measurement of electromagnetic and GW emission from GW170817 (Abbott et al. 2017a , 2017b ), which revolutionized our understanding of a wide range of physics related to merging neutron stars. Accurate sky localization and the rapid release of GW candidates are crucial for multi-messenger observations of astrophysical events. The former is achieved through a global network of GW observatories: the two Advanced LIGO detectors in the United States (Hanford and Livingston) together with the Advanced Virgo detector in Italy have been able to pinpoint the sky location of nearby GW candidates to areas smaller than 50 square degrees. The latter is achieved through fast and efficient data analysis pipelines. During the third observing run (O3) of Advanced LIGO and Advanced Virgo, low-latency GW alerts were publicly available within minutes through the Gamma-ray Coordinates Network (GCN) 4 4 https://gcn.gsfc.nasa.gov / and on the GW-candidate event DataBase (GraceDB). 5 5 https://gracedb.ligo.org/ For compact binaries, the information released in Open Public Alerts (OPA) included 6 6 https://emfollow.docs.ligo.org/userguide/index.html : the false-alarm rate (FAR) estimated by the search pipelines, the inferred sky position and distance of the source (Singer Price 2016 ; Singer et al. 2016a ), the probability of astrophysical origin with classification according to different mass regions (Kapadia et al. 2020 ), and the probability of neutron star matter content (Foucart et al. 2018 ; Chatterjee et al. 2020 ). Rapid and public information release enabled every astronomer with access to telescopes or neutrino detectors to search for counterparts to the GW candidates (Andreoni et al. 2019 ; Antier et al. 2020 ; Coughlin 2020 ; Coughlin et al. 2020 ; Graham et al. 2020 ; Hussain et al. 2020 ; Kasliwal et al. 2020 ; Vieira et al. 2020 ). Preliminary OPAs in O3 were generated automatically when low-latency GW search pipelines reported an effective FAR below either 1 per 2 months for the matched-filtering searches or 1 per year for the unmodeled burst searches. 7 7 https://emfollow.docs.ligo.org/userguide/analysis/index.html#alert-threshold This FAR threshold targets an overall astrophysical purity of 90% (Abbott et al. 2020a ), hence some candidates will likely originate from instrumental artifacts. GW candidates recognized as the result of an instrumental