While the rest of the forum is still trying to define for themselves the difference between Ni and Ne, I'd like to do a bit of discussing about Si tertiary.
Si-tertiary is a characteristic of the INFP and INTP types, that is, those that have Ne-auxiliary. The tertiary of any type will be the exact opposite of the auxiliary of the dominant type.
Si-tertiary behaves differently depending on which of the two types is dominant. If Fi is dominant, then the Si-aux will show itself in ways that expresses the personal values (or as Carl Jung would say, the subjective values) of Fi. When I say personal or subjective values, I mean that these values were thought up by the valuing subject, not that they are subjectively random or arbitrary. This is in contrast to the Fe type for whom values are objective, socially derived and conforming to the values of the external individual or group.
The healthy INFP will be socially charming, outgoing (although in a self-adjusting way) as well as politically and socially conservative. This helps lend this subjectivist type a feeling of internal structure as well as a way to support his internal life through externalization of personal values, that is, a virtuous goal to achieve or cause to support. Thus there will be a sense of group or brand loyalty with this INFP, but not of the kind that expects loyalty in return in the healthy INFP. Since Si is a tertiary type, it won't be highly developed even when the INFP is healthy. So they will be loyalists to a fault, more-or-less self-defeating at it, only far less so than the unhealthy INFP. And so Si tertiary will cause the INFP to keep "taking it on the chin" and behaving like a lovable doormat long past the point where loyal devotedness should have ended. But the healthier the INFP, the more self-sacrificing he is in a way that makes him or her heroic in the eyes of others, although the healthy INFP remains ever humble, and humbled.
The average INFP is still charming and outgoing, but a certain manipulativeness is seen in this type. Having lost a certain amount of self-acceptance, this trait arises from a desire to be accepted by others. Average INFPs can use their innate understanding of how other people tick in ways that are less than healthy. Having lost self-esteem for whatever reason, the average INFP begins looking for self-worth in the eyes of others. The manipulativeness of the average INFP is a form of control over others which is enabled by Si-tertiary. The latter also arises to aid the INFP as he or she seeks security in the tribe or external cause. This is an anxious type, and manipulating (controlling) others while being other-determined keeps the anxiety at bay.
The unhealthy INFP has begun to lose all self-acceptance while at the same time demanding acceptance from others. Si-tertiary begins to turn toward outright hostility as a form of controlling others and away from the mere manipulativeness of the average INFP.