Save value from a table

You say you want to "save" a choice. As the first reply pointed out there is no way to have anything like a global variable in DAX. However, you can put the possible selections into a separate table that has no relationships to the table you have your data in. This is an example of the DAX pattern "Parameter Table" https://www.daxpatterns.com/parameter-table/  

Then when the user selects a value from the parameter table list, it will be available for you to use in your measures, and you can use it to lookup the corresponding value in your data table by using a function like LOOKUPVALUE(). Since the parameter table is no related to your data table any selection the user makes on the data table rows will not change the value selected in the parameter table.