Means Test Income = CMI = Currently Monthly Income = Income received in the 6 CALENDAR months prior to the CALENDAR month in which you file divided by 6; i.e., today is Nov. 23, if I file today my CMI equation would look like this: (total aggregate gross income from May 1 thru Oct 31) /6 = CMI
THIS DOES NOT CHANGE throughout your case!! This is only used in the means test to determine whether a presumption of abuse arises when you file a chapter 7. If your expenses or income change, then those changes can be the basis for the UST's action to object to the chapter 7 under 11 USC 707(b)(3) as abuse. Though, I do not believe you are under any obligation under 11 USC 521 to amend your schedules for post-petition activity.
In a chapter 13 it is used to determine whether you are committing all your disposable monthly income to the plan. However considering SCOTUS's 2010 decision in In re Lanning, if you expenses change or your income changes, those changes can be considered when determining whether you are committing all your disposable monthly income to the plan.