I believe 1 unit subjects like Studies of Religion 1 give out bands 1-6. Only Extension subjects give out E1 - E4's.
To the original poster, bands correspond to descriptions of what students can do i.e. knowledge and skills. So each year, due to differences in test difficulty/ability of cohort/nature of the questions asked etc the raw mark cut off for a certain band will change. In other words, it's too hard to say what raw mark would be needed to align to a band 5 as this will change from year to year. Plus, we'll never know our raw marks.