what is CV_FM_RANSAC_ONLY ?

edit

calib3d.hpp says

#define CV_FM_LMEDS_ONLY  CV_LMEDS
#define CV_FM_RANSAC_ONLY CV_RANSAC
#define CV_FM_LMEDS CV_LMEDS
#define CV_FM_RANSAC CV_RANSAC

so seems to be useless for now

I would expect that CV_FM_RANSAC_ONLY computes rough fundamental matrix and filter outliers, while the CV_FM_RANSAC performs fundamental matrix computation using RANSAC algorithm to find rough fundamental matrix and then perform robust fundamental matrix estimation using only inliers from previous stage. This gives you better result but needs more time.