Seems that it was a unit raised to provide a garrison for Zeeland; hopefully this link works...

https://books.google.com.au/books?id=GDVUAAAAcAAJ&pg=PA12&lpg=PA12&dq=Zeeuwsch+Legioen&source=bl&ots=wyWhgEhgAh&sig=ACfU3U14ALO-KsAjAD_hakOdDY6J0P5quw&hl=en&sa=X&ved=2ahUKEwiu0c-8lqzlAhWK6XMBHe4-AQ8Q6AEwA3oECAkQAQ#v=onepage&q=Zeeuwsch%20Legioen&f=false

There are other mentions found in Google books. There seems to have been, by the way, a bit of a pattern of the Dutch raising local legions of this form...but it seems that legion was particularly associated with the 5th...

George