1.5.2.1 Use of Implementation-Defined Language Features

Note that conforming code may rely on particular implementation-defined values or features. Also note that the requirements for conforming code and conforming implementations do not require that the results produced by conforming code always be the same when processed by a conforming implementation. The results may be the same, or they may differ.

Conforming code may run in all conforming implementations, but might have allowable implementation-defined behavior that makes it non-portable code. For example, the following are examples of forms that are conforming, but that might return different values in different implementations:

 (evenp most-positive-fixnum) =>  implementation-dependent
 (random) =>  implementation-dependent
 (> lambda-parameters-limit 93) =>  implementation-dependent
 (char-name #\A) =>  implementation-dependent

1.5.2.1.1 Use of Read-Time Conditionals