The default keynote file is contained in the root directory of the library: C:\ProgramData\Autodesk\RVT 2016\Libraries\South Africa\RevitKeynotes_Metric.txt
Remember to set Windows Exoplorer to show hidden files and folders to be able to see the relvant libraries.
The Key Code consists of 5 base digits. The code structure allows for any alpha-numeric entry, although the default code is restricted to numeric entries.
The first two numbers denote the division. 100 divisions are possible: 00 through 99. Division 00 is commonly nominated as one that can house custom (company specific) codes.
Digit 3 denotes sub-divisions. Nine are possible: 1 through 9.
The last two digits denote the sub-sub-divisions, 01 through 99. They are nominally structured in intervals of 10. This allows one the opportunity to up to nine other sub-sub-divisions inbetween if future developments require this
When individual items are indicated within the sub-divisions, they are indicated by two digits after a decimal. The first decimal is a letter: A through Z, the next a number, 1 thorugh 9. When 9 is not enough, a digit is added (red arrow above).
If a company is of the intent to use the existing default to base their codes on, and if they would like to add their own codes into the existing structure, then the would have to add their own item modifiers after the decimal point to differentiate their items from the rest. More than three digits may be utilised after the decimal point.
If they need to create divisions, sub-divisions or sub-sub-divisions, they may also have to be denoted uniquely (using letters instead of numbers).
The alternative to using the defult Revit keynotes is to look to the CSI and AIA Standards.