Refer to figure.
1. According to EASA AIROPS - ANNEX I - Definitions:

• Take-off Decision Point (TDP) means the point used in determining take-off performance from which, an engine failure having been recognised at this point, either a rejected take-off may be made or a take-off safely continued.
• Take-Off Distance Available (TODAH) in the case of helicopters means the length of the final approach and take-off area plus, if provided, the length of helicopter clearway declared available and suitable for helicopters to complete the take-off.
• Take-Off Distance Required (TODRH) in the case of helicopters means the horizontal distance required from the start of the take-off to the point at which take-off safety speed (V_TOSS), a selected height and a positive climb gradient are achieved, following failure of the critical engine being recognised at the TDP, the remaining engines operating within approved operating limits.
• Rejected Take-Off Distance Available (RTODAH) means the length of the final approach and take-off area declared available and suitable for helicopters operated in Performance Class 1 to complete a rejected take-off.
• Rejected Take-Off Distance Required (RTODRH) means the horizontal distance required from the start of the take-off to the point where the helicopter comes to a full stop following an engine failure and rejection of the take-off at the take-off decision point.

2. According to CAT.POL.H.205 - Take-off (PC1): The helicopter is assumed to accelerate down the FATO (runway) outside of the height-velocity (H-V) diagram:

• If the helicopter has an engine failure before TDP, it must be able to land back on the FATO (runway) without damage to helicopter or passengers.
• If there is a failure at or after TDP the aircraft is permitted to lose height — providing it does not descend below a specified height above the surface (usually 15 ft if the TDP is above 15 ft).