I could not understand very well the "two layers" mentioned in the below help document regarding rule-set list assembling.

According to the document all rulesets are belong to the second layer, then what is the first layer ?

Can anyone explain the "two layer" concept?

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prhelp/rule-/rule-application/general.htm

Application form

Completing the Definition tab

About the RuleSets arrays

The Definition tab lists four sets of RuleSets :

• Application RuleSets
• Branches (with their associated branch RuleSets)
• Component and shared RuleSets
• Production RuleSets

The order of the entries within these lists affects the assembly of a user's RuleSet list and the operation of the rule resolution algorithm.When assembling the RuleSet list at sign-on, the system works from the top of each list down. RuleSets appear in the following order, in two layers.

• The operator's personal RuleSet, containing rules the operator has checked out, comprises the top of the second layer. (This RuleSet is not specified on the Application rule form. Operators have a personal RuleSet only if the Allow Rule Check Out? check box (on the Security tab of their Operator ID data instance) is selected.
• Branch RuleSets form the next lower group in the second layer.
• Application RuleSets form the next lower group in the second layer.
• Component and shared RuleSets, if any, form the next group of the second layer.
• Standard RuleSets provided by Pegasystems, that start with the five-character prefix "Pega-" (not the four-character prefix "Pega"), appear at the bottom of the second layer.