11.13.7.2 The current package for Lisp operations

When evaluating or compiling an entire buffer, the Editor uses in-package forms as they appear in the code. For any code that precedes the first in-package form, or when there is no in-package form, the code is evaluated or compiled in the primary package.

When evaluating or compiling a region of the current buffer (as opposed to all of it), the Editor uses in-package forms as they appear in the region. For any code that precedes the first in-package form of the region, or when there is no in-package form in the region at all, the Editor searches for the previous in-package form in the buffer. If this is found, it determines the current package, otherwise the primary pacakge is used.

When evaluating or compiling a definition, and for operations such as symbol completion at the cursor point, the Editor searches for the previous in-package form in the buffer. If this is found, it determines the current package, otherwise the primary pacakge is used.