Tags names are arbitrary lower case alphanumeric strings, that don't contain special characters. Stick to lower-case letters, digits, underscores and hyphens. Avoid special characters like /\!@#$%^&*()|{}~<> and punctuation characters.
Examples of partition tags for a transaction header include:
Examples of allocation tags for transaction detail items include:
The second item above shows a list of two allocation tags (i.e. financial.project_101 sub_company_a.project_101). All other examples listed show one allocation tag.
Notice that an allocation tag is actually a path, with two components separated by a "." (inserted by NewViews). The first part of the path is the partition tag (inherited from the transaction header), and the second part is the allocation tag (optionally selected per detail item). Together they uniquely identify the tag.
Tag names are important and you should try to think it through before you set it all up. For example, looking at the tag names used above, one might be tempted to use short partition tag names:
A middle ground to the examples shown above might use partition tag names:
The most important point is that you establish a consistent naming protocol. Finally, tag names can be changed at any time, the only caveat being that all transactions using the tag will be reindexed, and this may be a long duration task.