Naming convention: Population v. ``Has population''...renaming??
indeed sometimes this wiki uses "Has population" and sometimes "Population". This should be consistent. I thought that I tracked all of them but obviously I did not. I recommend to use a more distinct naming scheme following the expected context.
The example the Berlin just explains that two distinct properties have to be created in case you would like to cater for the expected context. Both properties take Berlin as a value, but the meaning conveyed by the property is different in both cases. The symbol "↔" may best be seen here as a replacement for "compared to".
In case you want to avoid having two different properties you would just call it "Capital". Both properties mentioned above will probably never be used on the same page, so doing "Capital" is arguably a valid solution, too. I personally follow the two properties scheme here since this proved to be the better solution for me. Again, there are other opinions around, too.