so this was something that happened to people without means of supporting themselves who wanted to move?
Anyone might carry with them a certificate stating that their own parish of settlement would support them if they fell on hard times and needed relief whilst in another parish.
As has been suggested, she was apparently on her way from London towards (
versus) ‘Postill’ when she gave birth.
It’s possible that she was being removed under the poor law from a London parish to Dorset. Parishes were always keen to remove unsupported pregnant women (whether widowed or unmarried) to another parish, if they could lawfully do so, to prevent the ongoing costs of maintenance falling on their local rate-payers.
If that is what was happening here (speculation only), ‘Postill’ should have been her late husband’s parish of settlement.