'Salic' derives from the Salic Franks on ancient Earth whose law code mandated Agnatic succession, that is in the male line only. This is the default rule of succession in the Barrayaran common law frequently circumvented by the Counts' recognized right to choose their successor. Undoubtedly there have been numerous cases of sister's sons or daughter's sons being so recognized and anything done twice on Barrayar creates a 'tradition'.
The succession of the Emperor Dorca the Just, a Vorbarra on his mother's side rather than his father's, provides the necessary precedent to disregard the Salic rule in regards to the Imperial Succession, or so some argue. Others feel differently, including most emphatically those whose female imperial descent would put them in the line of succession, like Aral and Miles Vorkosigan and Ivan Vorpatril.
Behind the scenes
There has been commentary from the author regarding Barrayar's ban on Salic Descent for the Imperial Throne (Camp Stool). See 106679.html for an example.