Offset quadrant shower trays are an elongated version of their quadrant siblings. The extra length basically provides more elbow room and thus a more comfortable shower in the resulting space.
Crucially when selecting an offset quadrant you need to understand the 'handing' aspect of the tray required. Each size will be available in a LH and RH version depending on which corner you install the shower in. Lookout for the handing diagram in each product which will demystify which handing you require.
Offsets generally start at 900x760mm in size and go right up to 1200x900mm. There are also some larger dedicated products out there that are made specifically for some curved shower doors.