According to the Citrix documentation1, burst capacity allows an instance to use more bandwidth than its dedicated bandwidth when there is spare bandwidth available on the SDX appliance. The priority parameter determines which instance gets the spare bandwidth first. A higher priority value indicates a higher priority. Therefore, the administrator can configure burst capacity of 1GB with priority 10 on the instance that requires more bandwidth, to meet traffic requirements.
1: Citrix Virtual Apps and Desktops 7 Deployment and Administration - Configure burst capacity