B.B. Warfield explains the three imputations of Christian theology

"In the developed theology thus brought into the possession of the Church [during the Reformation], three several acts of imputation were established and expounded. These are the imputation of Adam's sin to his posterity; the imputation of the sins of His people to the Redeemer; the imputation of the righteousness of Christ to His people. ... But in each and every case alike imputation itself is simply the act of setting to one's account; and the act of setting to one's account is in itself the same act whether the thing set to his account stands on the credit or debit side of the account, and whatever may be the ground in equity on which it is set to his account. That the sin of Adam was so set to the account of his descendants that they have actually shared in the penalty which was threatened to it; and that the sins of His people were so set to the account of our Lord that He bore them in His own body on the tree, and His merits are so set to their account that by His stripes they are healed, the entirety of historical orthodox Christianity unites in affirming." (B.B. Warfield, "Imputation," from "The Works of Benjamin B. Warfield," vol. IX, pp. 302-303, published by Baker)

Lorraine Boettner: Arminianism limits the atonement more severely than does Calvinism

"When the atonement is made universal its Inherent value is destroyed. If it is applied to all men, and if some are lost, the conclusion is that it makes salvation objectively possible for all but that it does not actually save anybody. According to the Arminian theory the atonement has simply made it possible for all men to co-operate with divine grace and thus save themselves — if they will. But tell us of one cured of disease and yet dying of cancer, and the story will be equally luminous with that of one eased of sin and yet perishing through unbelief. The nature of the atonement settles its extent. If it merely made salvation possible, it applied to all men. If it effectively secured salvation, it had reference only to the elect. As Dr. Warfield says, 'The things we have to choose between are an atonement of high value, or an atonement of wide extension. The two cannot go together.' The work of Christ can be universalized only by evaporating its substance.

"Let there be no misunderstanding at this point. The Arminian limits the atonement as certainly as does the Calvinist. The Calvinist limits the extent of it in that he says it does not apply to all persons (although as has already been shown, he believes that it is efficacious for the salvation of the large proportion of the human race); while the Arminian limits the power of it, for he says that in itself it does not actually save anybody. The Calvinist limits it quantitatively, but not qualitatively; the Arminian limits it qualitatively, but not quantitatively. For the Calvinist it is like a narrow bridge which goes all the way across the stream; for the Arminian it is like a great wide bridge which goes only half-way across. As a matter of fact, the Arminian places more severe limitations on the work of Christ than does the Calvinist." (Lorraine Boettner, Limited Atonement)