Okay, so the Dim statement is used to declare a variable. Declaring a variable is considered to be best practice when writing VBA, or pretty much any code for that matter. Most languages won't even let you not declare one. VBA will allow it, but almost nobody does it. This is because when you don't declare a variable, VBA assigns it a data type of Variant, which while it can hold the greatest number of values out of all the data types, it is the least efficient, slowest, and usually results in errors unless you are intentionally declaring something as Variant (for instance, a lot of arrays tend to be declared as Variant). In this case, the data type I chose was Integer, which can hold signed numeric values up to ~32,000 in magnitude. Since I know that both i and j will never come close to hitting that cap, it's one of the most efficient data types I can use in this scenario. Additionally, it prevents me from accidentally trying to assign a value like a string of text to that variable. If I tried to say that i should = "test", VBA would error out with a type mismatch error. If you put the words "Option Explicit" at the very top of your code, VBA will require that you declare all of your variables before it will run the code. I personally do this with every single bit of code that I write because declaring variables has so many advantages. Look up Data Types and Variable Declaration for more information on this.
The For i = 1 to 6 loop is used because your original code indicated that you wanted to call 6 different routines. By using the variable i instead of hard-coding the values, I saved a lot of space and made the code easier to both read and debug. Additionally, I noticed that there are several instances where you referenced a cell with a row number of 1 to 6 in sequential order (specifically, K1:K6). By using the variable, I was able to replace the row number in those cell references with the variable, eliminating the need to call them 6 different times.
Regarding the secondary loop based on the variable j, I noticed that there was a pattern to the cells that you were comparing your "K1:K6" cells to. You started at cell C21, and then just moved 6 cells down. Instead of referencing 5 different cells, I decided to use the secondary loop to allow VBA to compare the "K" cell to the correct "C" cell, one cell at a time. The math is based on starting at cell C21. So 6 * 0 + 21 is 21, 6 * 1 + 21 is 27, 6 * 2 + 21 = 33, and so on. Instead of trying to compare the value in "K#" to all of the "C#" cells simultaneously, this does it sequentially. As soon as it finds that "K#" is not equal to any one of the "C#" cells, it quits comparing the "K#" cell and moves to the remainder of the code.
The reason I used Application.Run instead of Call is because that's the syntax required in order to run a procedure based on a variable name. If you type Application.Run, followed by a space, you'll see a list of parameters that you can pass into it. The first one is the name of the procedure you want to run, and the rest are arguments that you can pass to that procedure. You didn't indicate that you were passing any variables to a procedure, so the only argument I needed was the name of the procedure to be called. Since the name of the procedures follows a logical pattern, I was able to save a lot of space (and lessen the likelihood of errors) by using a variable name instead of hard-coding it 6 different times. However, since the next procedure you call after your K#_2 is named the same in both occurrences of your original code ("Remove_List"), it makes more sense to simply call that one by hard-coded name instead of assigning it to a variable. Variables are essential for good code, however, if you can get away with not using one, I would at least consider doing so whenever possible.
Finally, the Exit For statement after the call to the "Remove_List" procedure is there to prevent the j loop from attempting to repeat everything it just did in case it were to find another "C#" cell that did not equal the the "K#" cell. Everything after that is just there to close loops and shut the code down.
Couple little notes, the original Case statement has seemed weird to me from the beginning. Since you mentioned that the lowest value it can be is 4, I would just change it to Case 4 To 18 and be done with it, that's your safest bet. Also, the "If .Value = Range("C" & (6 * j + 21)) Then" line should actually be "If .Value <> Range("C" & (6 * j + 21)) Then". That was a typo in the original code. The final code should look like this:
Code:
Sub NestedIfs()
Dim i As Integer, j As Integer
Select Case [C15]
Case 4 To 18
Select Case [D6]
Case "X", "Y", "Z"
For i = 1 To 6
With Sheets("Sheet2").Range("K" & i)
If [C15] = .Value And .Offset(, 1) = "" Then
For j = 0 To 4
If .Value <> Range("C" & (6 * j + 21)) Then
Application.Run "K" & i & "_2"
Call Remove_List
Exit For
End If
Next j
End If
End With
Next i
End Select
End Select
End Sub
