There are definitions of “enterprise” and “business” in the OECD Model Convention, which do not exist in the UN Model Convention. They are included in the OECD Model as an attempt to ensure that, when Article 14 (Independent Personal Services) was removed from the OECD Model Convention, all situations previously covered by it would be covered by Article 5 (Permanent Establishment) in combination with Article 7 (Business Profits).The UN Tax Committee recently decided to retain Article 14, so the same situation does not apply for the UN Model Convention.
However, it also agreed to have an alternative set of provisions and Commentary for those wanting to follow a similar course to that of the OECD Model Convention, in which case, some similar definitions would be likely.