
Explanation:
Box 1: CALCULATE
Start with CALCULATE and use a SUMX.
CALCULATE evaluates an expression in a modified filter context.
Box 2: SUM
Box 3: TOPN
TOPN returns the top N rows of the specified table.
Box 4: [TransactionDate]
TOPN Syntax: TOPN(<n_value>, <table>, <orderBy_expression>, [<order>[, <orderBy_expression>,
[<order>]]...])
The orderBy_expression: Any DAX expression where the result value is used to sort the table and it is evaluated for each row of table.
Reference:
https://docs.microsoft.com/en-us/dax/topn-function-dax