Preliminaries are the costs of running the project as a whole rather than any measured work item: site management and supervision, accommodation and welfare, cranage and hoisting, temporary services, security and the like.
They are priced over the planned duration, which is why time is money in a literal sense: when a project runs longer, prelims keep being incurred, and extended or thickened prelims are usually the largest head of a prolongation claim.
Proving them requires records of the resources actually maintained on site through the delay period, which is another argument quietly won or lost by daily records.