IF returns one value when a condition is true and another when it's false: IF(Product[List Price] >= 500, "Premium", "Standard"). Leave out the last argument and IF returns a blank.
SWITCH compares one value with a list: SWITCH(Sales[Channel], "Online", "WEB", "In-store", "STORE", "OTHER"). The last argument is the result when nothing matches.
For ranges, write SWITCH(TRUE(), condition1, result1, condition2, result2, else). SWITCH returns the result of the first true condition, so order matters: test the highest band first.
In a calculated column, the condition sees one row. In a measure, it sees the result of other measures in the current filter context. Check blanks explicitly with ISBLANK when a store or month has no data, or the label will say something false.