diff --git a/Custom-campaigns.md b/Custom-campaigns.md
index 87d4e18..89442d3 100644
--- a/Custom-campaigns.md
+++ b/Custom-campaigns.md
@@ -53,6 +53,12 @@ A FOB a a capturable point that can spawn ground units but not aircraft. A FOB i
 
 The group name of the unit is used to name the FOB.
 
+FOB defenses function similarly to airbase defenses and their spawn positions are determined by placing the same units. The only difference is that the chance for a group to be generated are determined as follows:
+
+*Between 2 and 6 defense groups are generated.
+*The first group has 1/2 chance of being a SHORAD and a 1/2 chance of being a garrison.
+*All further groups have a 1/3 chance of being a SHORAD and a 2/3 chance of being a garrison.
+
 #### Importance
 
 Control points can be assigned an "importance", which determines the proportion of their forces that the AI commander will assign to the control point. This value is set using the `Periodicity` property of the control point's warehouse. This applies to airbases only.