"Y" means yes, or a splice occurs at this level. "N" means no, or no splice occurs at this level. "T" means temporary, which means that no splice was specified at this level but a splice is necessary due to a change in column characteristics such as shape - if changes are made to the column such that the characteristics of the column don't change at this level, the temporary splice will be cleared and this will revert to a non-spliced level.