[exim] ACL, transport, & SpamAssassin

Hello Exim Users,

I have a few question re: Exim operation specifically ACL and transport functionality and how these interoperate with SpamAssassin.

In reading the Exim documentation, it is clear that SpamAsassin can be called by Exim at the desired time by using ACL control. Then, it seems that SpamAssassin will hand back a SPAM score & report so that Exim can then take the information and move forward with message processing. With that in mind, here are my questions:

1) Are Exim ACL and Transport modes mutually exclusive or can they work in tandem?
2) Is there any other method by which SpamAssassin can work with Exim or is it always done via ACL calls?
3) SpamAssassin has a feature whereby the email message subject line is rewritten with desired text. I also note that Exim seems to have a similar option discussed in the Transport section. Are these one in the same?
4) Can the subject line rewrite be used when SpamAssassin is called by Exim from an ACL call?

My knowledge and understanding of Exim is limited (I just learned about it earlier today) so my apologies up front for any misuse of terms and/or nomenclature. I also apoligize for any misunderstanding and ignorance re: Exim in general.

Thanks in advance for any feedback you can provide...