The Level-1A product is a simple concatenation of Level-0 data with a Header, which could easily be reversed back to Level-0. Level-1A remains in a binary format and is not in HDF. The Level-1B format groups like data together.
This file is the percent water surface coverage file used for the IMERG products in binary format. It is on a 0.1°x0.1° grid as REAL*4 (3600,1800). The orientation is the same as the IMERG data. The file displays percent water surface coverage with 100% = all water and 0% = all land.