Titles that are 'limited as to parcels' are for sites where the area and dimensions of the title are not guaranteed. The notation of whether the site is limited is usually found at the top of the title.
Sites that are limited to parcels are generally older subdivisions where a proper survey was never undertaken. Sites may end up being much smaller once the formal survey occurs, affecting land-use matters such as density and bulk and location controls.
Generally, if subdivision is to occur, a surveyor will need to survey not only the entire site but also the surrounding sites in order to gather evidence as to where the boundaries are. The surveyor will use fencing, buildings, spouting (worst-case scenario!) and driveways to assist in determining official boundary positions.
Always be cautious about any types of applications (land-use or subdivision) involving sites that are limited as to parcels, given the site boundaries and area are not guaranteed.