A leveling or numbering scehme is referenced within content through the numPr element, which itself refers to the num within the numbering part. The num is actually an instance of the abstract numbering scheme. This referencing of instances of abstract numbering schemes allows re-use of an abstract scheme so that variants of a single scheme can be defined without repeating the entirety of the scheme. This is the function of an overrride--it defines a variant of an already-defined scheme by simply overriding the particular properties of one or more levels of the scheme that are different. An override is specified with the <w:lvlOverride> element within the definition of a numbering instance (num) in the numbering part.
Below is the definition of an abstract numbering scheme, followed by an instance of the scheme with id=1, wich does not contain any overides. Following that instance is another instance which references the same abstract scheme but also specifies an override of the abstract definition for the first level by adding a top border to the level. Either instance could be referenced in a paragraph of content by simply referring to the desired num.
There is just one attribute:
Specifies the numbering level of the abstract numbering scheme to be overridden.
There are two element:
Specifies the appearance of the numbering level definition that is being overridden. See Defining a Particular Level.
Reference: ECMA-376, 3rd Edition (June, 2011), Fundamentals and Markup Language Reference § 17.9.6.
Specifies the number with which the level to be overridden should begin. It is used when the level starts or restarts. It has a single attribute val, which is the number. Note that the actual format will depend on the format specified by the numFmt element. See Numbering Format. E.g., <w:startOverride w:val="2"/>.
Reference: ECMA-376, 3rd Edition (June, 2011), Fundamentals and Markup Language Reference § 17.9.27.