A stable vector generally improves append performance (because you never have to move the data), allows storing data that can't be moved, and allows for appending while iterating. The downside is slightly worse performance for random lookups (since the data is fragmented across multiple segments) and higher memory usage (depending on implementation).

This "semistable" vector appears to only do the "allow appending while iterating" part, but still maintains a contiguous buffer by periodically moving its contents.