From the introduction:
We, the citizens of the Enclave, who do believe in the Enclave as the legal continuation of United States of America, rule that we will do everything that our country, its government, or citizens may need to achieve our goal of resurrecting the United States of America once again
-- Constitution of The Enclave, First Paragraph
Enclave expansion is recommended either for high level (12 and above) player characters form the Wastes or can be played from the very beginning (level 1) when using player characters that are Enclave citizen.
You should also be aware that this is a high-tech add-on and it introduces many cyberpunk ideas which can sometimes not fit into your post nuclear world but in my opinion when used in moderation it provides good gaming opportunities.
You have been warned.