This section is from the book "Popular Law Library Vol7 Equity Jurisprudence, Trusts, Equity Pleading", by Albert H. Putney. Also available from Amazon: Popular Law-Dictionary.
"There is also a class of cases in which the defendant may refuse to make a discovery as to particular charges contained in the bill, although a demurrer could not have been sustained as to the relief which the complainant intends to found upon those charges. Those, however, are cases in which the discovery asked for would tend to criminate the defendant, or subject him to a penalty or forfeiture, or would be a breach of confidence which some principle of public policy does not permit, and where the complainant may be entitled to the relief sought, upon the matters charged in the bill, although the defendant is not bound to make a discovery to aid in establishing the facts. But where the same principle upon which the demurrer of the truth of certain charges in the complainant's bill is attempted to be sustained is equally applicable, as a defense to the relief sought by the bill, the settled rule of the court is that the defendant cannot be permitted to demur as to the discovery only, and answer as to the relief. This general rule is equally applicable to the case of a plea; and the defendant cannot plead any matters in bar of the discovery merely, when the matters thus pleaded would be equally valid as a defense to the relief." 2
1 This classification is in the main that followed in Shipman on Equity Pleading.
2 Brownell vs. Curtis, 10 Paige, (N. Y.) 210.