it’s just the number of candidate cells which are sent to the target MSC when the serving BSC detects a HO to an external cell.
The HO CMD message may contain several candidate cells. The MSC will attempt to get a TCH on the 1st cell of the list. If the first cell is congested, then it will try with the second one, and so on.
By putting only 1 candidate cell in the message, you might delay the HO, if the target cell is congested : the BSC would have to wait for the reject from MSC, and then propose the second cell. That is a loss of time.
Proposing 6 cells at once is quite bad because if the 5th best candidates are congested, then the MSC will test the 6th candidate, which might probably a terrible candidate. If the 6th has a TCH “free” then the MS will have to go there. That might be dangerous.
As you understand, this parameter is useful only if the external target cells are congested !
Usually, 2 or 3 cells is plenty enough. If none of them is available, then you might as well jeep the MS in the current serving cell.
If the external cells are located in a dense area (multiband cells, microcells, etc), then choosing 6 candidates looks “less” dangerous.