Hi,
The Maximum length for the namespace is 16 characters.
You can have any namespace with this limitation however as a standard, the name of the namespace must be concise and must contain only authorized characters in accordance with XML naming rules.
Identifiers must not begin with numeric characters.
Certain namespaces are reserved for descriptions of the system entities required for the operation of the Adobe Campaign application:
xtk: concerning platform system data,
nl: concerning the overall use of the application,
nms: concerning delivery (recipient, delivery, tracking, etc.),
ncm: concerning content management,
temp: reserved for temporary schemas.
You can go through below link to know more on this.
Introduction