we can install windows enterprise and it will show the message "activate windows" with watermark in the bottom right corner of the screen, but after join the pc to the domain, windows will become activated.
Sounds like KMS activation. You could consult your system admin for an exact answer. If your company uses KMS activation, don't worry, your machine will keep in Enterprise activated state.
How KMS works
without having to enter a license key, a KMS software product automatically looks for a local KMS server during the installation. The software activates itself for 180 days if a KMS server has been found. Every week the software contacts the KMS server to reset this timespan back to 180 days. When the KMS server can not be reached for 180 days you get a grace period of 30 days. Within this 30 days you get a warning message and you have to reconnect to our KMS server. Otherwise the software reduces its features to a minimum until you reconnect to our KMS server.