Reeflex Pro Camera and ReeXpose apps utilize Reeflex-designed algorithms to produce computational long exposure photography.
This works very differently than how long exposure photography is usually produced with a regular DSLR camera, and provide incredible results with ease and without needing ND filters. The apps in fact take multiple pictures and stacks them together for creating a long exposure photo, a process often called Frame Averaging.
In the app, the shutter speed (SS) is related to the sensor and is translatable to the shutter speed in a DSLR. While the capture duration is the total time during which the app keeps acquiring frames which are then merged together.
As an example, if you would set a SS of 1s and a capture duration of 10s, the app will acquire 10 separate photos (each acquired with 1s shutter speed) and will stack them to produce the final result.
You can see the capture duration as the number of frames that the app acquires. The longer the duration, the more frames are averaged with subsequent more “blur” in the picture.
Here, it is important to notice that in Motion Blur mode, the level of exposure is not affected by the capture duration (i.e a 2 min photo will be almost as bright as a 3 min photo). This is also why the apps can produce long exposure without ND filters.
The shutter speed controls the exposure of the single frame. Setting a slower shutter speed will result in brighter pictures, regardless of the capture duration.
Note: the shutter speed of any iPhone device is currently limited to 1 second by the manufacturer. There's no way one can go beyond that, hence the reason for the need of computational photography to obtain long exposure photography.