Each Major lists the units which you must do in order to complete that Major sequence.
The recommended sequence is a good guide. It is recommended you do all those subjects in the sequence, because it will cover the 1st year units for most of the Majors, ie. Bio 1 & 2 and Chem 1& 2 are all required units in most every Major.
It's also recommended that in 1st year you do 2 Science alternatives, and again in 2nd Year. You can do any listed under the Alternatives in B Science (General Science). 100 level = 1st year workload, 200level = 2nd year workload etc. so it's up to you. However, there is a rule that during your 1st year, you must do at least one unit from four different areas of Bio, Chem, CompSci, Geo, Maths, Physics. Bio and Chem is covered, so you'll need at least one unit each from two other areas.
Then you have 2 electives in year1, 2 in year2 and 4 in year3, totalling. It depends on how strict this course is, but you should be able to choose any of your electives from any area. Look for electives under the relevent other degree. Forensics doesn't have a Major of its own it seems but this doesn't stop you from doing its electives. Look under B Science (Forensic Science); there's a Forensic Biology list of units.
As for Environment and Medicine (I assume Anatomy and Physiology), there's a Major sequence, and a sub-major sequence for each. This will depend on how in-depth you'd like to be in each area, Major sequences are more in-depth than sub-Majors; and whether or not you'd have enough elective choices to do them. As an example, to complete the Environmental Science Major, you do Bio 1 and Chem 1, then Microbio 1 OR Microbio 2 (you can do both but just one is required), plus 3 more units in that list. Since Bio 1 & 2 and Chem 1 are in the recommended sequence, and so is Biochem1 and Microbio 1 & 2... it means you'll use one elective for Environmental Science. This leaves you with 7 more electives, which you can use to complete another Major or sub-Major.
I don't know what the Four Biology Core Units means though.
I hope this has been slightly less confusing than the handbook.
-andy