I have a detail table DT1 with items... Name,unit price,cost etc...
Consider this as an invoice, I want another field to be the sum of costs of detail table DT1...
I can go to the editbox of this field and change the value... Of course if I use "Update all sum fields" button, it returns to the correct value... but, I think this should be like expressions... I mean, when I define a VAT expression then I can't change the VAT value in the editbox...
I believe this should happen with summary fields too...

I don't know if this is a bug or if this happens for a reason so I didn't post to the bug report section

It should update the summary field value automatically without user clicking the button. We will check it.
The summary field is different from the expression field. The summary field stores the actual value. It allows user to edit this field is on purpose for the value adjustment.

I think I didn't make it clear for you...So let me re-phrase it...
Field F1 of DT1 : Cost of each item...
In DT1, I get the sum(F1) and it updates automatically...

I use the sum(F1) as the value of another Field F (of the main table). When I insert F in the layout I get an editbox but I can change the value... I think this is wrong and it has no meaning since I don't change any of the F1 values in DT1... This should work as if F was an expression... It gets the actual value but there is no meaning to be able to change it.

Anyway, this is not really important...

Thanks for the clarification. We may add an option to handle that in the next version.