Hi,

Security mechanism in odoo is implemented using access control, record rules and groups.

Access Control

Managed by the ir.model.access records, defines access to a whole model.

Each access control has a model to which it grants permissions, the permissions it grants and optionally a group.

Access controls are additive, for a given model a user has access all permissions granted to any of its groups: if the user belongs to one group which allows writing and another which allows deleting, they can both write and delete.

If no group is specified, the access control applies to all users, otherwise it only applies to the members of the given group.

Available permissions are creation (perm_create), searching and reading (perm_read), updating existing records (perm_write) and deleting existing records (perm_unlink)

Record Rules

Record rules are conditions that records must satisfy for an operation (create, read, update or delete) to be allowed. It is applied record-by-record after access control has been applied.

A record rule has:

• a model on which it applies
• a set of permissions to which it applies (e.g. if perm_read is set, the rule will only be checked when reading a record)
• a set of user groups to which the rule applies, if no group is specified the rule is global
• a domain used to check whether a given record matches the rule (and is accessible) or does not (and is not accessible). The domain is evaluated with two variables in context: user is the current user’s record and time is the time module

Global rules and group rules (rules restricted to specific groups versus groups applying to all users) are used quite differently:

• Global rules are subtractive, they must all be matched for a record to be accessible
• Group rules are additive, if any of them matches (and all global rules match) then the record is accessible

This means the first group rule restricts access, but any further group rule expands it, while global rules can only ever restrict access (or have no effect).

You can see this information in Odoo Documentation: https://www.odoo.com/documentation/13.0/reference/security.html

Thanks