  Power electronic interfaced devices progressively enable the increasing
provision of flexible operational actions in distribution networks. The
feasible flexibility these devices can effectively provide requires estimation
and quantification so the network operators can plan operations close to
real-time. Existing approaches estimating the distribution network flexibility
require the full observability of the system, meaning topological and state
knowledge. However, the assumption of full observability is unrealistic and
represents a barrier to system operators' adaptation. This paper proposes a
definition of the distribution network flexibility problem that considers the
limited observability in real-time operation. A critical review and assessment
of the most prominent approaches are done based on the proposed definition.
This assessment showcases the limitations and benefits of existing approaches
for estimating flexibility with low observability. A case study on the CIGRE MV
distribution system highlights the drawbacks brought by low observability.
