The real wordsTheoryUnverified
The measure
MeasureA formula evaluated at query time, once per cell of the visual, under whatever filters are then in force. Nothing is stored.
It works in filter context: there is no current row, only a current selection. Put a measure in a chart of sales by city and it is evaluated once per city, each time seeing only that city's rows.
Because it is not stored, it responds to every slicer, filter and cross-highlight the reader touches. Because it is not a column, it cannot go on an axis or in a slicer.