Imputation Line - getLines

Hi.
I hope you can help me to understand the following fragment of code from method getLines in Imputation line.

// fetch all work stored in database for this assignment
foreach ( $workList as $work ) {
if (($work->idAssignment and $work->idAssignment == $elt->idAssignment and !$work->idWorkElement and !isset($elt->_idWorkElement)) or
(!$work->idAssignment and $work->refType == $elt->refType and $work->refId == $elt->refId) or
($work->idAssignment and $work->idAssignment == $elt->idAssignment and $work->idWorkElement and isset($elt->_idWorkElement) and $elt->_idWorkElement == $work->idWorkElement)) {
$workDate=$work->workDate;
$offset=dayDiffDates($startDate, $workDate) + 1;
if (isset($elt->arrayWork [$offset])) {
$elt->arrayWork [$offset]->work+=$work->work;
} else {
$elt->arrayWork [$offset]=$work;
}
}
}


It seems that you collect in the same work object the amount of work from work table having
a) a given assignment and no _idWorkElement
b) no assignment , a given reference
c) a given assignment and a given idWorkElement

Now, I'm to create a) just as a real work referred to an activity, b) as the real work for a ticket, but I'm not able to create c) .
How can I ?
Furthermore, how can I create two or more lines in work table that will be added in the same imputation line ( as in the previous fragment of code ) ?
Conditions appear incompatible, but if not, what will happen at save ?
Thank you for your help

Thank you for your explanation.
Now I'm able to create a work line with assignment and idWorkElement.
But at the moment I could believe that the condition to add to different amount of work as in the code line
$elt->arrayWork [$offset]->work+=$work->work;
will never happen.
It's true ?

Ok, I try to explain.
In the code you test isset($elt->arrayWork [$offset])
If true you add the new work amount to the previously stored.
I cannot understand how this condition can happen : you should find two lines in db having the same date and matching one of the three conditions above.
Is this possible ?