Asset can be whatever you want. It's just a term to determine where a post belongs. With CB Activity 6.0.0 this will be easier since activity streams are just normal stored in the database objects that you can even specify the view access level for. For now though yes you need to make a Joomla menu item or a field with your custom assets. I recommend maintaining the following structure for an asset.
Code:
TARGET_ORIGIN . TARGET_TYPE . TARGET_ID . OBJECT_ORIGIN . OBJECT_TYPE . OBJECT_ID . VERB
Example as follows.
profile.30.field.20
This is broken down as follows.
TARGET_TYPE = profile
TARGET_ID = 30
OBJECT_TYPE = field
OBJECT_ID = 20
An asset for a CB GroupJive group would be like the following.
groupjive.group.10
Which breaks down as follows.
TARGET_ORIGIN = groupjive
TARGET_TYPE = group
TARGET_ID = 10
In this case since there's no explicit object specified then the object is just whatever the target is.
This structure is CB Activity 6.0.0 compatible. So when creating custom usages it's ideal to follow this, but it's not required. So for example all of the following are fine.
global.cars
global.movies
movies.scary
custom.1
custom.2
custom.3