Manuscript editing as opposed to developmental editing

From the Chicago Manual of Style - 15th Edition

Manuscript editing, also called copyediting or line editing,
requires attention to every word in a manuscript, a
thorough knowledge of the style to be followed,
and the ability to make quick, logical, and
defensible decisions. It is undertaken only when a
manuscript has been accepted for publication. It may
include both mechanical editing and
substantive editing. It is distinct from
developmental editing (not discussed in this manual),
which addresses more radically the content of a work, the
way material should be presented, the need for more or
less documentation and how it should be handled, and so
on. Since editing of this kind may involve total rewriting
or reorganization of a work, it should be done—if
needed—before manuscript editing begins.

Mechanical editing
Copyediting involves two processes. The first, being
concerned with the mechanics of written communication,
is known as mechanical editing. It refers to consistency in
capitalization, spelling, hyphenation, table format, use of
abbreviations, and so forth; correctness of punctuation,
including ellipsis points, parentheses, and quotation
marks; the way numbers are treated; consistency
between text, tables, and illustrations; citation format;
and other matters of style in the sense described in 2.52.
Mechanical editing also includes attention to grammar,
syntax, and usage at the most basic level. Such editing
may include either preparing an electronic file according
to the requirements of the production process and the
medium in which the work will be published or, if the
editing is done by hand, marking up the hard copy for a
typesetter.

Substantive editing
Substantive editing deals with the organization and
presentation of existing content. It involves rephrasing for
smoothness or to eliminate ambiguity, reorganizing or
tightening, reducing or simplifying documentation,
recasting tables, and other remedial activities.
In general, no substantive
work should be undertaken without agreement between
publisher and editor, especially for book-length works; if
major substantive work is needed, the author should be
consulted and perhaps invited to approve a sample before
the editing proceeds, to avoid delays later.

Discretion in substantive editing
A light editorial hand is nearly always more effective than
a heavy one. An experienced copyeditor will recognize and
not tamper with unusual figures of speech or idiomatic
usage and will know when to make an editorial change
and when simply to suggest it, whether to delete a
repetition or an unnecessary recapitulation or simply to
point it out to the author, and how to suggest tactfully
that an expression may be inappropriate. An author’s own
style should be respected, whether flamboyant or
pedestrian. Although fact checking is not usually expected
of a manuscript editor (at least in most university
presses), obvious errors, including errors in mathematical
calculations, should always be pointed out, and
questionable proper names, bibliographic references, and
the like should be verified or queried. All manuscript
editors should know whether the publishers or journals
they work for have strict policies on terms relating to race
or ethnicity, sex, age, and other areas of potential bias or
whether individual discretion is allowed.