There is no statutory definition of fair dealing and, in a court of law, use would be judged on a case-by-case basis.
‘Fair dealing’ includes the use of works for illustration for instruction and quotation. Works made available to the public can be used for criticism and review.
In essence, ‘fair dealing’ means use is limited to only the minimum necessary amount of content for its purpose (reasonable and appropriate to the context) and use does not negatively impact on the market for the original works. Copying should not harm the copyright holder, financially or otherwise.