I'm trying to put a conditional format on fields in a sub-summary layout part.

The condition is a 'formula' condition.

The formula tests whether a summary field (count of records being summarized) = 1.

FM does not apply the conditional formatting.

Is this a limitation of the conditional formatting?

Check this thread and particularly Comment's solution:

here .

You need to use the GetSummary() function, so that Filemaker knows it needs to count only the records in the group.

Doesn't seem to work.

The formula I tried looks like:

getsummary(<>,<>)

The 'breakField' is the 4th (lowest) level in a sort. The condition is not applied

The field I'm trying to apply the condition to is in a report sub-summary section. Does that make a difference?

<> displays properly in the sub-summary section. But apparently that value is not used in the conditional format formula??

<> displays properly in the sub-summary section. But apparently that value is not used in the conditional format formula??

That is correct. Only summary fields themselves respond to their layout placement. If you reference a summary field in a calculation (such as in conditional formatting formula), you have to specify explicitly which one of the many possible summary results you want. The object to which the conditional formatting is applied is irrelevant, what matters is the formula.

It's hard to tell why it doesn't work for you. One possible reason could be that your breakfield is coming from a related TO; GetSummary() doesn't like that. Also make sure that your sub-summary part definition, your sort order and your GetSummary() calc are all using exactly the same breakfield.