In most jurisdictions, a plaintiff or claimant must complete a statement of claim or such similar document in which are set out the allegations of facts they intend to prove. In the aggregate, those allegations of fact, if proven at trial, must suffice to give rise in law to the relief being claimed.
The claimant or, as the case may be, the plaintiff would have to provide sufficient detail in the claim so that the defendant knows what the allegations of fact are; otherwise, that person cannot defend. In a statement of defence, it is imperative that the defendant admit or deny each of the allegations of fact set out in the statement of claim.
The nature and extent of particulars depends on the nature of the claim. If, for example, trespass is alleged in the claim, the allegation of fact must make that claim out. Similarly, for breach of contract, the claim would set out the material terms of the contract and the details of the alleged breach.
While, in regards to each of the above examples, it would not be enough to allege that trespass or breach of contract occurred, nor is the claimant required to set out the allegations of fact considerable detail; just enough for the defendant to know the case he or she is facing. As further example, a statement of claim in regards to personal injury would set out the date and time of the injury and the general circumstances of it as well as allegations of negligence and details as to the allegations of injuries.
In CAT, Justice McNair of the Federal Court wrote:
"The object of particulars is to enable a party to know the case of his adversary so as to avoid surprise.
"Particulars are ordered more freely than in earlier times for the days of trial by ambush are now gone. Courts today are insistent that pleadings define with clarity and precision the issues to be tried. Where general allegations are made which in earlier times might escape scrutiny, in these days particulars will usually be ordered. Courts have drawn a distinction between particulars required before pleading and those required before trial. The purpose of particulars required to be delivered before pleading is for the intelligent pleading by the opposite party. As to particulars before trial, a party is generally entitled to any particulars required to properly prepare his case for trial."
In Fairbairn, Justice Ferguson of the Ontario Court of Appeal wrote:
"Particulars are ... ordered for several purposes: the define the issues, to prevent surprise, to enable the parties to prepare for trial and to facilitate the hearing."
In a criminal trial, note the words of Justice Pennell in R. v Canadian General Electric:
"The function of particulars in a criminal trial is twofold. Primarily their function is to give such exact and reasonable information to the accused respecting the charge against him as will enable him to establish fully his defence. The second purpose is to facilitate the administration of justice."
- CAT Productions Ltd. v. Macedo, 1 C.P.R. (3d) 517
- Fairburn v Sage,  2 D.L.R. 536 and 56 O.L.R. 462
- R. v. Canadian General Electric Co. (No. 1), 17 C.C.C. (2d) 433 (1974)