Help with evaluating some DAX code

IF statements work from the top in so it check 1, the 2 etc:

It startts with 1 and IF 'Cases[ERT Start Flag Date] is not blank it goes to section A which starts the 2 check, if it is blank it gos to B and returns a BLANK().

Hi ArchStanton 

The explanation from jdbuchanan71 is perfect. Regarding your second question, you can use variables in this way:

Days ERT to Outcome = 

var _startFlag= ISBLANK ( 'Cases'[ERT Start FlagDate] )
var _endFlag= ISBLANK ( 'Cases'[ERT End FlagDate] )

IF (
    NOT ( _startFlag),
    IF (
        ISBLANK ( 'Cases'[formofauthorityreceiveddate] ),
        DATEDIFF (
            IF (
                'Cases'[Created Date] > 'Cases'[ERT Start FlagDate],
                'Cases'[Created on],
                'Cases'[ERT Start FlagDate]
            ),
            IF ( _endFlag , NOW (), 'Cases'[ERT End FlagDate] ),
            DAY
        ),
        DATEDIFF ( 'Cases'[ERT Start FlagDate], 'Cases'[ERT End FlagDate], DAY )
    ),
    BLANK ()
)

However, as you can see, it doesn't make much sense. Variables avoid repetitive calculus, but in this case, you don't have repetitive pieces