  A recently explored interesting quantity in AdS/CFT, dubbed 'residual
entropy', characterizes the amount of collective ignorance associated with
either boundary observers restricted to finite time duration, or bulk observers
who lack access to a certain spacetime region. However, the previously-proposed
expression for this quantity involving variation of boundary entanglement
entropy (subsequently renamed to 'differential entropy') works only in a
severely restrictive context. We explain the key limitations, arguing that in
general, differential entropy does not correspond to residual entropy. Given
that the concept of residual entropy as collective ignorance transcends these
limitations, we identify two correspondingly robust, covariantly-defined
constructs: a 'strip wedge' associated with boundary observers and a 'rim
wedge' associated with bulk observers. These causal sets are well-defined in
arbitrary time-dependent asymptotically AdS spacetimes in any number of
dimensions. We discuss their relation, specifying a criterion for when these
two constructs coincide, and prove an inclusion relation for a general case. We
also speculate about the implications for residual entropy. Curiously, despite
each construct admitting a well-defined finite quantity related to the areas of
associated bulk surfaces, these quantities are not in one-to-one correspondence
with the defining regions of unknown. This has nontrivial implications about
holographic measures of quantum information.
