In the last decade, there has been much research on diaper topsheets that have an opening, which leads to a void space to receive or store BM.  The focus has mainly been to improve the positioning of the opening against the wearer's body and in proximity of the anus of the wearer, for example by use of elastication means around the opening in the topsheet. 
Many types of materials have been suggested as topsheet, including both hydrophobic and hydrophilic topsheets, such as for example described in U.S. Pat.  No.  4,662,877 and U.S. Pat.  No.  4,990,147. 
The inventors have now found that it is highly desirable that the BM, once received through the opening, is not visible for the person changing the article, i.e. that the topsheets masks the BM.  The topsheet further has to be such that it has a high air-permeability, and that it is flexible and not too thick, in order to provide comfort to the wearer.  The various hydrophilic and hydrophobic topsheet materials suggested in the art do not provide such masking.  Other materials used in parts of a diaper that may mask BM, such as a materials generally used as backsheets, are too thick or not air permeable enough to be used as comfortable topsheet. 
The invention thus seeks to develop a diaper, which has a topsheet with one or more opening, which is such that it masks the BM received through the opening (s) and present underneath it. 
The inventors found that this can be effectively achieved by using in an absorbent article a topsheet with a uniform and small pore size distribution and at least and an inner surface (i.e. facing the backsheet) that has a very low surface energy.  Surprisingly and unexpectedly, the combination of these properties provides excellent masking of the BM, whilst the use of either one of the properties alone, does not provide the required masking benefit. 
The preferred topsheet of the diapers of the invention ensures that the appearance of at least the inner surface of the topsheet remains about the same before and after contact with BM, e.g. a white, non-transparent topsheet remains like that with BM underneath and in contact with it.  Thereby, the BM is effectively hidden for the person changing the diaper. 
Surprisingly, the inventors have found that the use of this topsheet of the diapers of the invention, which has a very low surface energy and a uniform porosity, also helps reduce the leakage of BM, and in particular very runny BM, from the void space underneath the topsheet, back through and onto the topsheet and to the wearer's skin.  This has thus also been found to be a highly effective way to reduce leakage from the diaper.  Also, the topsheet of the diapers of the invention provide an effective rewet barrier for runny BMs and therefore protect the skin from BM and the irritants in it. 
It should be noted that it is known in the art to apply chemicals to a topsheet of a diaper to make it more hydrophobic or hydrophilic, for example to control urine absorption or repellency, but it has not been recognized in the art that it is beneficial to significantly reduce the surface energy of the topsheets with one or more large opening, to aid the masking of BM in the diaper and to reduce leakage.  Also, it has not been realized that the combination of a very low surface energy and a uniform pore size distribution is required to provide masking of BM received in the diaper.