Thanks a bunch for your explanation. Greatly appreciated!
As for your first point, I ran the model with TRA_centered * REL_centered, and observed (almost) the same coefficients. BUT the new plot (attached) is quite different (particularly for SOICAP=3) from that with TRA_REL as a multiplicative term. How could this be possible?
Coefficients (when I used TRA*REL):
SIZE_centered 0.8434
ASSIM_centered 0.5333
PRIOR_centered 7.1007
TRA_centered 0.7901
REL_centered 0.6990
TRA_centered:REL_centered -0.5274
Coefficients (when I used the multiplicative term):
SIZE_centered 0.7723
ASSIM_centered 0.7808
PRIOR_centered 6.9093
TRA_centered 0.7389
REL_centered 0.7383
TRA_centered:REL_centered -0.5212
Anyway, I think the new plot is less ambiguous when it comes to the direction of the interaction effect. For example, at the uppermost row of the plot, the probability of SOICAP increases with REL up to a point where it decreases as a result of higher TRA. So we can conclude that TRA and REL are negatively related to each other, both from the coefficient and the effect plot.
As you suggested, I used interplot to illustrate the marginal effects of REL and TRA. Berry et al.'s paper was quite useful. As far as I understand from the marginal effect plots, the marginal effect of REL on SOICAP is positive and significant at the minimum value of TRA, and negative but not significant at its maximum. This is also true if we look at the inverse plot with REL as the moderator. So the conclusion would be that 1) The interaction between REL and TRA is negative since the slope of the lines in the marginal effect plots are negative, but it is not significant since the line crosses zero. 2) The marginal effect of REL on SOICAP is positive and significant for low-medium values of TRA, and vice versa. 3) Even if the marginal effects turn non-significant for the high values of each predictor variable, we can still accept the support of negative interaction since the distribution of moderator variables in the marginal effect plots show that the percentage of observations falling into the insignificant area is low. Are these statmenets correct?
New plot (TRA*REL instread of the multiplicative term)
Marginal effect plot when TRA is the moderator
Marginal effect plot when REL is the moderator