  The far-field channel model has historically been used in wireless
communications due to the simplicity of mathematical modeling and convenience
for algorithm design. With the need for high data rates, low latency, and
ubiquitous connectivity in the sixth generation (6G) of communication systems,
new technology enablers such as extremely large antenna arrays (ELAAs),
reconfigurable intelligent surfaces (RISs), and distributed
multiple-input-multiple-output (D-MIMO) systems will be adopted. These enablers
not only aim to improve communication services but also have an impact on
localization and sensing (L&S), which are expected to be fundamentally built-in
functionalities in future wireless systems. Despite appearing in different
scenarios and supporting different frequency bands, such enablers share the
so-called near-field (NF) features, which will provide extra geometric
information conducive to L&S. In this work, we describe the NF features,
namely, the spherical wave model, spatial non-stationarity, and beam squint
effect. After discussing how L&S see NF differently from communication, the
opportunities and open research challenges are provided.
