A solo sector is a Stellar sector within the Star Imperium and later under the United Planets that is composed of a single galaxy, unlike Intergalactic sectors which contain two or more galaxies. 

Often a single galaxy lacking it's own satellite galaxies, a solo sector is often relatively isolated from other galaxies.  And 10 is an exception (it is a satellite galaxy of the Andromeda Galaxy despite being in different sectors) since it contains Alfheim, capital of the most dominant government in the known universe.