It is a classical result of Sobolev spaces that any $H^1$ function has a well-defined
$H^{1/2}$ trace on the boundary of a sufficient regular domain.
We discuss its recent extensions given in \cite{TiDu16}
in some heterogeneously
localized nonlocal function spaces. The new trace theorems
are stronger and more general than the classical result. They
can be established essentially for all functions
having only square integrability
away from the boundary or
in any compact subset of interior domain. Yet, the heterogeneous
localization offers the necessary regularity precisely at the boundary
to have well-defined traces.
A consequence is that we may study associated
Dirichlet type boundary value problems,
as well as the coupling of local and nonlocal
equations through co-dimension-1 interfaces.
%Their derivations not only involve various extensions of classical inequalities but also
%require new techniques absent from traditional approaches.