When you use Bracket Orders and key in a Target and SL price (without trailing stop), you can modify your SL to any price you wish. However, if you add a trailing price to your SL, the way the system works is different. I've given three probable scenarios below:
1. Original order if you do not modify the SL price
Buy - 8320
original SL - 8300 / trail - 5 points
In this case the SL will be modified for every 5 point raise from entry price to match the trail. So the SL will move to 8305, 8310, 8315, and so on...
If SL is modified above the original SL price:
SL - modified to 8310 from 8300
Note that trail cannot be modified
In this case, when Nifty goes to 8325 the SL will trail back to 8305. SL will not trail from the modified price, instead it will consider the original price (8300) and trail to 8305 and 8310 (again) and so forth.
If SL is modified below the original SL price:
SL - modified to 8290 from 8300
Note that trail cannot be modified
In this case, if Nifty falls to 8315 the SL will remain at 8290 which is the modified price. If Nifty continues to fall without bouncing back, the modified SL will be hit which is 8290.
If the stock starts moving up and in this case goes to 8325, then the SL will change automatically to 8305 considering the original SL and trigger.