A 'Seeded' Blidline is a blank line of work with one 'seeded' trip on it.
For example, it may have a four day LAX on it Sunday to Wednesday. By default, this would mean that you would be guaranteed the 2 days off before and after, as LAX is a 'long range' destination with the requirement for 2 local nights off before and after.
In practical terms, even if you don't want to go to a particular destination, it can be very useful in getting specific days off that are required if you are otherwise not senior enough to achieve them otherwise or have no pre-booked leave etc.
As another tip, it is very worthwhile bidding for your sim checks. They have to give you a day off before a sim check, so bid for your sim check on a Monday, and you will get the Sunday off - you will be surprised at how many forget to bid for their sim check.
You can also bid for the time slot for your sim. So, depending on where you live and the traffic bid for early or late slots [normally the slot on day 2 is 4 hours earlier than on day 1].
Another tip, while I am at it, is to express a preference for your line check. It can be a way to get to a 'nice' destination that you can't achieve with your seniority!
[from a retired 744 captain]
/oops ... see that Wiggy has already responded .... hope this adds some extra value ...../