A "roving sub" assignment implies that you won't be in one classroom the entire day. I've "roved" for several classrooms at a single school. I've "roved" two different schools the same day to work a half day at each.
It seems that 2015 is the year of "perfect storm" conditions for substitute shortages. The number of teachers being out of the classroom has greatly increased for "Teacher Training" while the number of active substitutes teachers has not. I know a number of substitutes purposely cutting back work availability or leaving for one reason or another.
Last week the district office contacted me to see if I would be willing to do "roving assignments for the district office". I hadn't heard of this before so I had to ask exactly what I would be doing as a "district office roving sub".
I would be pre-reserved as a "District Office Roving Sub" for any days in the week I was willing to be available. If they didn't need me, I would still get paid.
If they did need me, the office would call after 07:40am with a list of schools and grade levels to pick from.
As an added bonus, being pre-reserved means that the automated system will not be calling at 05:30am. My wife especially likes this feature.