y8 asks: is there a concept like this?
Frontingliness or defrontability: the ability to front; a plural/system member able to front or more likely to front.
This would technically be the noun for the host, I guess. However, the host for me is the entirety of my fragmented personhood, and those who front are fragments of it manifesting momentarily. There are fragments (sysmembers) that are less likely to front (less defrontable alter/less frontingly headmate), as they stay dormant or latent (sometimes remaining extinct).
This experience is also tied to recovery, as the more analogous (micro)traumas (micro- in a psychoanalytic perspective) I experience, or the more I'm coped, the more defrontable or strong a specific alter is about to switch and stay there, or the more fused, borrowed, merged, or blended the members become, leaving the issues behind/to the past.