The symbol is a visual que to show that a table calculation has been applied to your measure.
Running total, % of total etc are all table calculations which are applied to your existing measure. You can think of this as a second calculation on top of you current one.
For example, in my view I have sum of sales and then a % of total based on the sum of sales. The Delta symbol just lets me know which one of the two fields has a table calculation applied:
More information on table calculations here:
The delta says it's a table calc.
A table calc is one that works across cells in a table. When you have SUM(Sales), that value is calculated on a cell by cell basis. But when you do WINDOW_SUM(SUM(SALES)), it sums up the sum of sales across the marks.
When you edit a table calc by selecting "Edit Table Calc" from the pill on the sheet, it is to tell tableau how to "walk" the table when doing the calc. For example, do you want to sum up sales within each Market for each year? Or should you sum up all the markets for each month? Settings get those different treatments for you.
Thank you Mavis and Joe