For the case of spinoffs it reflects the market as activities as the specific steps that have to be followed take place.
For example the spinoff of Leidos from SAIC in 2013. (I picked this one becasue I knew some of the details)
On September 9, 2013, the Board of Directors of SAIC, Inc.(Ticker
Symbol (NYSE):SAI) approved the following:
The separation of its technical, engineering and enterprise information technology services business through the distribution of
shares of SAIC Gemini, Inc. to stockholders. Each stockholder of
record of SAIC, Inc. as of September 19, 2013 (Record Date) will
receive one (1) share of SAIC Gemini, Inc. common stock for every
seven (7) shares of SAIC, Inc. common stock held by such stockholder
as of the Record Date. This distribution will be effective after
market close on September 27, 2013 (Distribution Date). After the
Distribution Date, SAIC Gemini, Inc. will be renamed Science
Applications International Corporation (New SAIC).
A one (1) for four (4) reverse stock split of the SAIC, Inc. common stock effective as of Distribution Date. After the Distribution Date,
SAIC, Inc. will be renamed Leidos Holdings, Inc. (Leidos).
Q 11: What are the different trading markets that may occur between
Record Date and Distribution Date?
A: Beginning two days prior to the Record Date of September 19, 2013
through the Distribution Date on September 27, 2013, there may be
three different trading markets available with respect to SAIC, Inc.
and the separation.
Stock Ticker – SAI (Regular Way Trading with Due Bills): Shares of SAI common stock that trade on the regular-way market will trade with
an entitlement to shares of the New SAIC common stock distributed on
the Distribution Date. Purchasers in this market are purchasing both
the shares of Leidos and New SAIC common stock.
Form of Stock Ticker –SAIC (When Issued Trading): Shares of New SAIC common stock may be traded on a “when-issued” basis. These
transactions are made conditionally because the security has been
authorized, but not yet issued. Purchasers in this market are only
purchasing the shares of New SAIC common stock distributed on the
Form of Stock Ticker – LDOS (Ex-Distribution Trading): Shares that trade on the ex-distribution market will trade without an entitlement
to shares of New SAIC common stock distributed on the Distribution
Date. Purchasers in this market are only purchasing the shares of
Leidos common stock.
So the stock price for New SAIC starts a few days before the record date of 19 September 2013, while LDOS (new name for the old SAIC) goes back much earlier. But the company didn't split until after the close of business on 27 September 2013.