referencing grid columns, finding selected records

[Sveta1978]

i have a grid. in the grid i have lines of data. how do i reference the particular columns in the grid to perform some calculations? Also, how do i tell when and what records are selected? datasource is a TEMPORARY table. as soon as records load, the table is blank...

its the DAX 4.0

Inventory Management --> Periodic --> Sales Release Order Picking screen
sales lines grid

Thank you!