I am designing dashboard, and have a filters panel on the side. What is the best way to reflect the user that the filtering is an 'AND' combination and not an 'OR' combination?
Filters exist of Filtertype and Attributes. For example: Filter is Size and attributes are 38, 39, 40, 41, 42, etc. Another filter might be color with attributes yellow, blue, red, etc.
If a user selects multiple sizes it will be absolutely clear to the user that this is OR operator without thinking of it this way. But it will be clear, that there will be no item which has two sizes. With color this might be more difficult. Since there might be items with more than one color. But in this case the attributes should be combined like red, red/yellow, yellow, green, etc.
If the user combines filters the expectation will always be that this is an AND operator. If a users chooses attribute 40 in filter size and attribute green in filter color she would not expect to find all items with size 40 in any color and all items in green in any size in the result. The expectation will be that the list shows green items in size 40.
Having said so. By grouping attributes in to filters the behaviour of the filters should be clear. If you have some attributes that should be combined as AND you should us them as filter with only just one attribute. For example 'show items on SALE only'.