To avoid confusing owners, CRF strives to keep its sail definitions in synch with those laid out in the Equipment Rules of Sailing (ref page 30, https://d7qh6ksdplczd.cloudfront.net/sailing/wp-content/uploads/2020/10/05151946/Equipment-Rules-of-Sailing-2021-2024.pdf ).  These can be summarized as saying that a spinnaker is a sail set forward of the mast whose mid width is equal to or greater than 75% its foot width, while a headsail is a sail set forward of the mast whose mid with is less than 75% of is foot length.  The ERS also say that a sail ‘set flying’ is one that does not have an edge attached to a stay.  A ‘tweener’ is then a headsail (mid with less than 75% of its foot length), and may be a  ‘headsail  set flying’ (HSF) because its luff flies free, and is not attached to a stay.  The Classic Yacht Racing Guidelines https://www.classicyachts.org/wp-content/uploads/2023/05/Classic-Yacht-Racing-Guidelines-2023.pdf that govern most CYOA events currently state under ‘Setting and Sheeting of Sails’ that all headsails need to be flown while ‘fully attached’ to a stay.  Since ‘tweeners’ are headsails and most have luffs that are built to be free flying (not attached to a stay) they cannot be used in events governed by the CYOA Guidelines.  At the same time, a big overlapping (often masthead) headsail whose luff IS FULLY ATTACHED to a stay can be used, as long as its hoist and tack location (often equal to ‘ISP’ and ‘TPS’) and its overlap are accounted for on the CRF certificate as ‘IG’ ,‘J’, and ‘LP’, and  as long as the stay (or l=uff rope) that supports this sail is kept in place and taut throughout the event.  The configuration cannot be changed from race to race or from day to day depending on the conditions, and NB that only one CRF rating configuration change is allowed in any one calendar year.