TCA Reference

I changed the namings as it avoids confusion with the existing eval method "unique" and "uniqueInPid".

$tcaFieldConf = array(
'type' => 'check',
'eval' => 'maximumRecordsChecked',
'validation' => array(
'maximumRecordsChecked' => 5
)
);

what you can do is this:

Say you have a checkbox (type=check) in a TCA table. Let's say we call it "Is important news" - the new option like configured above allows to set maximum of 5 records with a checkbox like this. When editing record #6, the checkbox on that record will be set to 0. You need to uncheck the field in another record first in order to set the checkbox on record #6.

I also provided a new array "validation" for the configuration, so "is_in" validation information could be moved there as well.

Note: This only works for checkboxes.

We have "maximumRecordsChecked" and "maximumRecordsCheckedInPid" for that option.