22.6.4 External formats and stream-element-type

The :element-type argument in open and with-open-file defaults to the value of *default-character-element-type*.

If element-type is not :default , checks are made to ensure that the resulting stream's stream-element-type is compatible with its external format:

1. If direction is :input or :io , the element-type argument must be a supertype of the type of characters produced by the external format.
2. If direction is :output or :io , the element-type argument must be a subtype of the type of characters accepted by the external format

If the element-type argument does not satisfy these requirements, an error is signalled.

If element-type is :default the system chooses the stream-element-type on the basis of the external format.

LispWorks User Guide and Reference Manual - 21 Dec 2011