Like backup job settings , Log shipping job also supports a backup strategy :

No strategy : If you  select this option , job will do transaction log shipping every time. 
Counter Strategy :   If you select this option , job will set up two counters for transaction log backup and differential backup . If a counter reaches the specified number , the job will select corresponding backup type to back up database . 
Size Strategy : If you select this option , job will record the size of transaction log and differential backup .If backup size reaches the specified bytes , it will select corresponding backup type to back up database .