Please take this explanation with caution as this comes from reading various Honda published papers and popular articles and then drawing some conclusions. I believe that a "wet" clutch connects the engine directly to the single speed transmission in the "direct engine drive" mode that is used at cruise above about 45 mph when the HV mode is selected. A "wet" clutch runs in the transmission fluid (like typical clutches in the usual automatic transmissions or the manual, multi-plate clutch in many motorcycle transmissions). The HV mode is in "parallel" when the engine clutch is engaged and the engine is directly connected to the transmission. The single speed transmission is geared fairly high similar to the over-drive gearing in a typical transmission. In this mode some additional torque can be supplied by the electric traction motor when required such as for climbing hills.
When there is a high torque demand, even over 45 mph, the clutch disengages and the system goes into a "series" HV mode where the engine mounted generator supplies the engine load and the traction motor gets power from both the battery and the generator. This "series" mode allows the engine to operate in a higher (and more powerful) rpm range than if the engine was directly connected to the single speed (over-drive) transmission. This mode often results in the "angry bees" sound as the engine is operating in a higher rpm range with more power output as compared to the rpm and power of the engine when directly connected to the transmission.
The Hybrid Accord has a similar power system but has a larger, more powerful engine than the Clarity. Thus, the control settings are different and the Accord engine can stay directly connected more of the time when there are higher torque demands. The control settings depend on the available engine power, the maximum power that can be drawn from the battery, and the maximum power that can be produced by the traction motor. Please note that the maximum power (amperage) that can be drawn from the battery is a major limit on torque. For example, in the case of a Tesla that can have the "ludicrous" mode, the power cables from the battery are upgraded to carry more amperage. Even so, the time that the maximum current can be drawn is limited by the ability to cool the battery and electrical system during high current flows. The Honda system is fairly conservation in torque output, current draw from the battery, and current input to the traction motor. Along with the Clarity's active battery and inverter liquid cooling systems, this makes for a more reliable power system.