The 'x' is one of the following letters...
s.. sentence case
u.. uppercase
l.. lowercase

and so on.

When you are using the online Mpc Editor, click the 'Vars' button. This will display the controls for inserting the correct syntax without having to remember the coding.

Select the Var you want to insert (it will turn green), then click the blue casing button to insert at the current cursor location.

In the "Mpc Pages" section of the Control Panel, you should be able to click "Edit Page" button and select the file in your Mpc folder that you want to edit. Is that not working ?

Once in the editor, click the Vars button and follow the info in my previous post.

Then click Save button to put the page back into your Mpc folder.

The editor does a little bit of formatting that may conflict with D.W. somewhat. If you want to put in the Var codes without using Mpc's editor, here are the codes (Var1 in these examples):

#var1:x# where x is as follows:
  t .. Title Case
  s .. Begin sentence
  u .. UPPER CASE
  l .. lower case
  h .. hyper-link (spaces become hyphens)
#var1# .. as is (no :x)