Lanczos4
"Lanczos resampling and Lanczos filtering are two applications of a mathematical formula. It can be used as a low-pass filter or used to smoothly interpolate the value of a digital signal between its samples. In the latter case it maps each sample of the given signal to a translated and scaled copy of the Lanczos kernel, which is a sinc function windowed by the central lobe of a second, longer, sinc function. The sum of these translated and scaled kernels is then evaluated at the desired points.
Lanczos resampling is typically used to increase the sampling rate of a digital signal, or to shift it by a fraction of the sampling interval. It is often used also for multivariate interpolation, for example to resize or rotate a digital image. It has been considered the "best compromise" among several simple filters for this purpose"(1)
Advice
The filter number at the end of the filter name is the width of the filter (4 pixels).
Note
Until the 0.9.4 release, the interpolation used in Siril is by default BICUBIC. This interpolation is not good enough for astronomical images because it increases fwhm a lot.
Since 0.9.5 release, the solution is to use Lanczos algorithm, but not alone because of halo around stars. The method for generating an improved rotation result is to use the Super Sampling technique.