What does it mean when there are two letters in the subscript next to a variable inside a summation equation?

Question

What does it mean when I have multiple subscripts inside a summation equation?

I have a formula with a summation equation that iterates over weekdays (i) and over device types (phone, tablet, desktop) (j). Inside theres a variable cost. Cost has this subsript: ij. What does ij mean?

I'll follow with an example and a screenshot to the formula

1. Is it the sum of the their cost?
2. Is it the product of their cost?
3. Or is it more like the sum of a filter

Example Phone (10€), Monday (20€), Phone on Mondays (3€)

1. 10€+20€=30€
2. 10€*20€=200€
3. Of the 20€ of Mondays 3€ are Phones (and/or) Of the 10€ of phones, 3€ are Mondays

Picture of the equation - what does cost ij mean