I guess this is a frequent question and I think the original documentation should be more detailed. It only says you can use tag or conditional line, but does not explain how to do it.
For example if you have 2 same elements one in 12h format another in 24h format, both in same place, you can use the tag as follows:
opacity of 12h element: 100-100*[IS_24H]
opacity of 24h element: 0+100*[IS_24H]
Or you can use condition line:
It can also be used to show/hide separate am/pm indicator