I'm not sure how well documented pit macros at iRacing. It seems to be something you need to search for as I wasn't able to find it in documents and tools. There is a thread on the subject in their forum, but it's not the easiest place to find things.
Pit macros allow you to define a pit strategy, which can be something as simple as taking repairs alone, or a combination of detailed changes. For example a certain amount of fuel, and tyres at certain pressures. The macros are all proceeded with a hash.
To clear fuel, windscreen and tyre checkboxes:
To clear the tyres alone:
Now you have a clean car, you can add whatever you desire.
To check the fuel box:
To add a certain amount of fuel, in this example 10 gallons:
To check the windscreen tearoff box:
Now we get into tyre macros.
To change the left front tyre:
If you want to have the tyre set to a certain pressure, in (p)si or (k)pa:
The same terminology is used for all four tyres, lf and rf at the front, and lr and rr for the rear.
You can also combine these instructions in one macro. For example:
#clear ws lf rf
That would clear the car, then check the windscreen tearoff box, and replace the front tyres.
Another command which may be useful, especially if more options are added later, is: