The source the ACTIVE profile's layer of this id pulls from.
Read the way the pull resolves it (offline.source.url, collectionId defaulting to the
layer id), so that what a pull tags and what a read compares are the same string.
Parameters
layerId: string
The layer.
Returns string|null
Its source key, or null when the active profile declares no pull source for it.
The source the ACTIVE profile's layer of this id pulls from.
Read the way the pull resolves it (
offline.source.url,collectionIddefaulting to the layer id), so that what a pull tags and what a read compares are the same string.