The <odoo> element can have a noupdate attribute, which is propagated to the ir.model.data records that are created when reading the enclosed data records for the first time, thus ending up as a column in this table.
When Odoo installs an add-on (called init mode), all records are written whether noupdate is true or false. When you update an add-on (called the update mode), the existing XML IDs are checked to see whether they have the noupdate flag set, and if so, elements that try to write to this XML ID are ignored. This is not the case if the record in question was deleted by the user, which is why you can force not recreating noupdate records in the update mode by setting the forcecreate flag on the record to false.
In legacy add-ons (prior to and including version 8.0), you'll often find an <openerp> element enclosing a <data> element, which contains the <record> and other elements. This is still possible, but deprecated. By now, <odoo>, <openerp>, and <data> have exactly the same semantics; they are meant as a bracket to enclose XML data.