Is this standard an extension of the WIS2 Topic Hierarchy?

[josusky]

Current draft of WME (WIS2 Monitoring Events) states that:

This standard is an extension of the WIS2 Topic Hierarchy

and this is then reflected also in "Table 2. WETprimary topic levels" where in the level 2 it has "Alphabetical version of the topic hierarchy".
Does it mean, that if the version of WTH changes to "b" because of, let's say, change in the ".../weather/prediction/forecast" sub-tree, do we need to change the version of WET (WIS2 Event Topic) too? Why should it if it does not use the deeper levels of WTH?