All toolboxes are inclusive of the Toolbox prior to them. If you load C, it will include everything in A and B.
If you loaded toolbox A, and are not in a State listed in the later toolboxes, and do not use .IRS LEVY, you do not need to get the later toolbox.  If your state is listed in a later toolbox and you use .IRS LEVY please use the later toolbox.
Tax changes for the following are in Toolbox 2012A:

* .E FICA (FICA - Employer share)
* .FICA (FICA - Employee share)
* .FUTA (Federal Unemployment)
* .FWT (Federal Withholding)
* .FWT NRA (FWT Non Resident Alien)
* .IRS LEVY (IRS Levy)

Release A includes updates to the following data items for state and local use:
* .IN COUNTY (Indiana Counties)
* .SDI CA (California Disability Insurance)
* .SWT CA (California)
* .SWT CT (Connecticut)
* .SWT DC (District of Columbia)
* .SWT HI (Hawaii)
* .SWT IL (Illinois)
* .SWT KY (Kentucky)
* .SWT ME (Maine)
* .SWT MN (Minnesota)
* .SWT NM (New Mexico)
* .SWT ND (North Dakota)
* .SWT VT (Vermont)

This document will be edited as more information is received. If your state is not listed for either Toolbox A , B or C, a Support Analyst can assist you with further information.
Toolboxes released after A include states that were later in getting their changes published.

The documentation posted on the website with the Toolbox will list which items are in it.
Items in Toolbox B are
.SWT DE (Delaware)
.SWT ID  (Idaho)
..SWT NY  (New York)
.SWT OK (Oklahoma)
.CTY YKRS (Yonkers)
.IRS LEVY

Items in Toolbox C are
.SWT MA (Massachusetts)'
.SWT MD (Maryland)
.SWT MO (Missouri)
.SWT OR (Oregon)
.SWT RI  (Rhode Island)

Items in Toolbox D are
.SDI CA (California sdi limit)
.SWT ID (Idaho)

Item in Toolbox E is
* .SWT MI (Michigan)

If you would like to set up an Optional Data Item to track all Health Care Benefits and Deductions together for the Affordable Care Act, see BB728983.  W2s will allow you to add up to 10 data items together for reporting purposes as will User-Defined reports, so a combined data item is optional in most cases.