In Quantum Information processing, we can extract entanglement from $n$-copies of a weakly entangled state to produce a fully or highly entangled states in $d$-dimensions, using the known distillation protocols.
However, there are states from which no entanglement can be extracted out using the known distillation protocols for no value of $n$. Such states contain bound entanglement.
For qubits, PPT is a necessary and sufficient criterion for separability. In higher dimensions, i.e qudits, there are states which satisfy PPT but are entangled and such states contain bound i.e non-distillable entanglement. I am not sure but I guess there are states which are NPT but bound entangled.
What is the deep physical reason behind the existence of bound entanglement?