Custom fields per pipeline is a new feature that allows to specify custom fields which are relevant only in single pipeline.
To set up up a custom field for a single Pipeline go to Settings>Pipelines>Custom Fields.
The dropdown field at the top indicates in which pipeline we are currently in and displays the custom fields in this pipeline.
As you can see global custom field are marked with globe icons, meaning they are visible on all pipelines. These cannot be edited from this section. Instead global custom fields can be created or edited in Settings>Deals>Custom Fields.
When adding a custom field on your Pipelines page it will only be added and visible on the selected pipeline.
If you would like to change a custom field from global to a specific pipeline this can be done by clicking 'Edit' on the custom field and changing the pipeline to 'All Pipelines'.