Owner-based versus criteria-based sharing rules
Key Takeaway
Owner-based sharing rules pick records by who owns them. Criteria-based sharing rules pick records by field values, so use them for records owned by high-volume users, who can't be in owner-based rules.
Sharing rules are automatic exceptions to your org-wide default sharing settings. They give users in public groups, roles, or territories more access to records they don't own. The two main types differ in how they pick records: owner-based rules go by who owns the record, and criteria-based rules go by the values in its fields.
What every sharing rule does
A sharing rule can only widen access. Like the role hierarchy, it can never be stricter than the org-wide default. You can create sharing rules only when the object's org-wide default is "Public Read Only" or "Private", because a "Public Read/Write" default leaves nothing to open up.
Every sharing rule answers two questions: which records to share, and who gets access at what level. The "Share with" setting names who gets access. The rule type changes only the first question. Both types share with public groups, roles, or territories.
Sharing rules apply to all new and existing records that match, and to both active and inactive users. They are reevaluated when group, role, or territory membership changes and when records are transferred. Large changes can take a while to recalculate.