An Ignored.t is an unusable value with special bin-prot converters. The reader reads the size and drops that much data from the buffer. Writing is not supported, however the size of t is kept, so bin_size_t is available.
This can be used in similar situations to Opaque.t. If instead of transforming a bin-prot stream, you are simply consuming it (and not passing it on anywhere), there is no need to remember the bin-prot representation for the bits you're ignoring. E.g. if you wish to extract a subset of information from a bin-prot file, which contains the serialized representation of some type T (or a bunch of Ts in a row, or something similar), you can define a type which is similar to T but has various components replaced with Ignored.t.