Numbers

Infinities and NaNs and are artefacts that help deal with the
inexactness of binary floating-point arithmetic. The semantics
dealing with infinities and NaNs, or the circumstances leading to
their generation are somewhat arbitrary. However, as most Scheme
implementation use an IEEE 754-conformant implementation [17]
of flonums, R^{6}RS uses the particular semantics from this standard as
the basis for the treatment of infinities and NaNs in the report.
This is also the reason why infinities and NaNs are flonums and thus
inexact real number objects, allowing Scheme systems to exploit the closure
properties arising from their being part of the standard IEEE-754
floating-point representation. See
section 11.6.6 for details on closure
properties.

R^{6}RS intentionally does not require a Scheme implementation to use
infinities and NaNs as specified in IEEE 754. Hence, support for them
is optional.

A distinguished -0.0 is another artefact of IEEE 754, which can be used to construct certain branch cuts. A Scheme implementation is not required to distinguish -0.0. If it does, however, the behavior of the transcendental functions is sensitive to the distinction.