There are three predefined profiles available for the stretched-coordinate PML boundary conditions: standard, stabilized and steep angle. They have appropriate settings for the PML parameters, such as kappa, sigma and alpha. You have some freedom to modify the number of layers according to your needs (this is typically a fine tuning of your results). If you want to explore the PML behavior modifying all the settings, you can do so in the “custom” profile, but this is an advanced setting; therefore, careful testing is necessary to make sure your simulation behaves properly.
The predefined profiles would work in most applications. Here is some quick advice on how to select the right profile:
Standard: Works in most cases, particularly when you have PML on all boundaries or PML combined with symmetric/anti-symmetric boundary conditions. If you are not sure what PML to use, this is a good starting point.
Steep angle: Use it when the PML is expected to absorb light at steep angles, usually this happens when you have periodic or Bloch boundaries on at least one boundary, possibly combined with symmetry/anti-symmetric boundary conditions for periodic structures. Note that even when the source is at normal incidence, light can reach the PML at steep angles, as illustrated here for light transmitted through a grating of nanowires:
Stabilized: Use it only in case of a diverging simulation due to the PML (see https://kb.lumerical.com/en/index.html?layout_analysis_diverging_simulations.html for details). With this setting your simulation will take longer because it has more PML layers. Also, stabilized settings can lead to divergences that would not be present if you used the other profiles. Therefore, this setting is not a good starting point for your simulations, only use it if necessary.