HP ProCurve Series 2810 Management And Configuration Manual page 103

instance spanning-tree network enables blocking of redundant links in one
instance while allowing forwarding over the same links for non-redundant use
by another instance. For example, suppose you have three switches in a region
configured with VLANs grouped into two instances, as follows:
VLANs Instance 1
10, 11, 12 Yes
20, 21, 22 No
Multiple Instance Spanning-Tree Operation
Instance 2
No
Yes
Overview
5-3