Solution : https://service.sap.com/sap/support/notes/36280 (SAP Service marketplace login required)
Key words : 
field reserves free background work processes, remaining free background processes free, remaining free batch work process, free background processing work processes, maintain work process distribution, -step job prioritization separate, free background work processes, background work processes high, reserve background work processes, background work processes reserved
Related Notes : 
       
| 39412 | How many work processes should be configured? | 
| 24092 | Distribution of background jobs on application server |