Fields in a custom mapping must be of the same or a similar type. You can’t sync a numeric field to a date field for example. When selecting fields in a custom mapping, the list on the other side will only show the fields that can be mapped, unavailable mappings are grayed out.
The best way to see if a mapping is possible is to check it out yourself in your own sync.
Sometimes a direct field mapping won’t work, but rules can make a sync properly work.
More information on this can be found over in our dedicated knowledge base article on why certain fields can't be mapped directly.
We also have an extensive article on how the feature of field mapping works.