What is the same
Both are compact 12-volt alkaline stacks used in remotes and transmitters. Pack labels such as 23A / MN21 / 8LR932 map to A23; 27A / MN27 / GP27A / L828 map to A27.
What is different
Diameter: A23 is about 10.3 mm across; A27 is about 8 mm — thinner. Forcing the wrong cell can damage the tray or leave poor contact.
Many garage systems that look similar use CR2032, CR2016, AAA or 9V instead — match the accessory model, not the brand logo on the opener.
Examples on this site
Genie GIT-1 and some Hampton Bay fan remotes name A23. CompuStar CM1000-K (R150) and Xianfeng YK remotes name A27 / 27A.
Do not use list
Do not substitute A23 for A27 (or the reverse) because both say 12V.
Do not buy “Galaxy A23 / A27” phone packs when the remote tray needs a miniature 12V cylinder.