Go to each product category, and select one of these values in the field
"Procured purchase grouping":

* *Standard grouping*: With this option, procurements will generate
  purchase orders as always, grouping lines and orders when possible.
* *No line grouping*: With this value, if there are any open purchase order
  for the same supplier, it will be reused, but lines won't be merged.
* *No order grouping*: This option will prevent any kind of grouping.
* *<empty>*: If you select nothing, default value set up in System
  settings will be applied.
* *Product category grouping*: This option groups products in the same purchase order that belongs to the same product category.

System default behaviour can be set up in System settings / Purchase / Procurement
Purchase Grouping
