This is in reference to the extent to which the hypochlorite compound used (as in swimming pools) generates Cl2 when
reacted with hydrochloric acid according to:

NaClO + 2HCl -----------> NaCl + H2O + Cl2

Thus, the chlorine is generated "in situ"... in this case.

Here is a "reference":

http://it.answers.yahoo.com/question/index?qid=2009020610293...

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Note added at 11 mins (2009-06-24 16:37:32 GMT)
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In the following reference, the term "theoretical active chlorine" is used:

http://www.halamid.com/IMG/pdf/TB_concentration_and_neutrali...

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Note added at 12 mins (2009-06-24 16:39:18 GMT)
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XXX active chlorine content
To start with, it is important to understand that XXX®
has a theoretical active chlorine content of 25.2%.
This can be explained as follow:
The reaction between Halamid® and iodide
liberates iodine:
C7H7SO2NHCl + 2 HI => C7H7SO2NH2 + I2 + HCl
C7H7SO2NHCl = Halamid®
This is comparable to the reaction of free chlorine
with iodide:
Cl2 + 2 HI => I2 + 2 HCl
The amount of active chlorine is expressed in terms
of elemental chlorine that would have similar oxidizing
power. Thus with Halamid®, one atom of chlorine (one
molecule of Halamid®) liberates as much iodine as two
atoms of elemental chlorine (Cl2).
So for the pure Halamid® (chloramine-T trihydrate,
M = 281.7), the active chlorine is:
2x(35.5/281.7) = 25.2%
(35.5 is the molar mass of Cl2)
This 25.2% active chlorine content is for the pure
trihydrate form. It can slightly vary depending of the
amount of crystalline water.
Therefore the Halamid® concentration in solution is
directly related to the active chlorine one. Active chlorine
being easy to determine, the main methods to measure
XXX® concentration are based on the active chlorine
measurement.

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Note added at 15 mins (2009-06-24 16:41:41 GMT)
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I would also add "content"... it is implied in the original:

so

** theoretical active chlorine content **

This is the contexnt based on the stoiciometry (as determined by molecular masses and coefficients in the equation). The actual amount of liberated chlorine might be slightly less... due to imputiry, losses, etc.

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Note added at 3 hrs (2009-06-24 20:10:52 GMT)
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Info: I am a chemist.

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Note added at 22 days (2009-07-16 20:23:45 GMT) Post-grading
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Welcome, Therese.