Heavy and light punctuation are distinguished mainly by practices regarding nonrestrictive modifiers, which heavy punctuation sets off by commas and light punctuation omits, so that with light punctuation, many nonrestrictive modifiers are punctuated the same way as restrictive. As long as the pattern is consistent, authors commit no outright error by populating documents with a greater or lesser comma density. While light punctuation is acceptable, even today's trend in fiction, better legal-writing practice affords less freedom. We can see why by asking what trade-offs characterize light and heavy punctuation practices. Heavy comma usage improves Clarity but impairs Concision, since Concision stands for less language, not only fewer words. Concision is information compression serving efficient comprehension, and a comma inserted sacrifices efficiency when the added punctuation doesn't change the message imparted. This trade-off of Clarity for Concision means the recipient spends slightly more time reading a heavily punctuated document but, receiving more guidance from the author, is less prone to comprehension error and confusion.
Despite the greater Clarity heavy punctuation supplies, lawyers usually use light punctuation. Distinguishing restrictive and nonrestrictive modifiers is one of the harder grammatical discriminations. Cognitive interference explains what makes the distinction difficult, even for lawyers. The restrictiveness of a modifier is a syntactic classification, but it parallels a similar dimension of meaning. Degree of restrictiveness in syntax comes in only two kinds, restrictive and nonrestrictive, whereas restrictiveness in the world is infinitely varied. Restrictiveness "as a matter of law," that is, syntactic restrictiveness, contrasts with restrictiveness "as a question of fact,” semantic restrictiveness. The continuous dimension of actual restrictiveness cognitively conflicts and interferes with the categorical grammatical distinction between restrictive and nonrestrictive modifiers. Trial attorneys, who dwell mostly in the world of fact, prefer writing with light punctuation, although they would rather read a heavily punctuated legal document. [See also "The Underestimated Comma."]