One of the columns of data for each record is the voter's 'status,' which can be active or inactive (along with several other categories that indicate the voter has been removed from the roll). For 'inactive' voters, the State Board of Elections classify them by this:
If a county board of elections has not had any contact with a voter for a period of two federal election cycles, then the voter will be sent a forwardable address confirmation mailing. The voter will be required to return the confirmation mailing within 30 days of the mailing. If the confirmation mailing is not returned by the voter within that time, or the mailing is returned by the postal system as undeliverable, then the voter’s record will be marked inactive in the voter registration database. Inactive voters are still registered voters. If an inactive voter presents to vote, the person will be asked to update his or her address with the board of elections. In the event that an inactive voter remains in this status for another two federal election cycles (meaning the county board still has no contact with the voter), then the voter will be removed as a voter in the county.*
Within the 6.4 million registered voters, 1.2 million, or 19 percent, are classified as 'inactive voters.' The following graphs give some sense of who these inactive voters are in the state.
* "Indian" could be either American Indian or Alaskan Native.