Could you please help me understunding how shedule works?

Analize this example from the wiki:

Schedule {
        Name = "ScheduleTest"
        Run = Full 1st sun at 23:05
        Run = Differential 2nd-5th sun at 23:05
        Run = Incremental mon-sat at 23:05
}

QUESTION 1: '1st' refear to the backup number (the first backup) or
the day of the week? or the day of the month?

I read the first line like:
"The first backup has to be full (so it doesn't matter the day it
begin). Each sunday do a full backup"

The second line...:
"Second and fifth backup has to be Differential" what about 'sun' ????

The third line:
"Make an incremental backup from monday to satturday"

I think that the second and third line overlap:
if sunday we make a full backup, monday (2th) should be done
differential (second line) but also incremental (third line).
QUESTION 2:I guess the first match is used and the other ignored (like
in iptables rules).

QUESTION 3: what means 'sun' ? If it's sunday, why is it used in
second line if it's already  specified in first line?

QUESTION 4: why is it usefull to make differential backups instead of
all incremental backup?

QUESTION 5: I read bacula user manual but found nothing about shedule
sintax. If you have any link, please paste it here.

Thank you.

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