"Sealed" means that probate was applied for in another jurisdiction from England/Wales.
Probate has to be applied for in each jurisdiction (e.g. property or shares in England?) so that the legal niceties can be observed.
My friend died a few years ago in the Isle of Man; but because she had inherited shares from her father, probate was applied in both IOM and England.
Granted in IOM; Sealed in England.
Other (legal) jurisdictions include Scotland, Northern Ireland, Isle of Man, Jersey, Guernsey and Republic of Ireland.