DAX TOTALMTD function and advice on using it.

Hi,
Can some one help or point me in the direction of information on TOTALMTD,

As far as I can gather the syntax is

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First does the date in the Totalmtd come from the 'sales' table or the CALENDAR table ? I've tried both and neither worked.
I've attached a two column table with some dates and figures, any help welcome as I seem to be misunderstanding
the basics.

update post

After a bit more experimenting I've found that

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does work, but I still don't understand TOTALMTD so still need some help.

Richard.