It's usual that GPT websites provide their users with some sort of affiliate program or referral system. A member is said to have been referred by other member when someone signs up "under" some other member's account. The collection of all users who have been referred under a given member is called the downline of said member. Likewise, the line formed by the referrers (and their referrers) of a said user is called a upline.
For instance, if UserB signs up under UserA and UserC and UserD sign up under UserB, there's a referral configuration such that:
The downline of UserA is formed by UserB, UserC and UserD.
The downline of UserB is formed by UserC and UserD.
The upline of UserB is UserA.
The upline of UserC is UserB->UserA.
And so on.
The number of members between two users in a referral line is called level.
So according to the above example:
The level 1 downline of UserA is formed by UserB.
The level 2 downline of UserA is formed UserC and UserD.
And so on.
Since a member can refer many users under his/her account, most downline structures tend to have a pyramidal or matrix like form, i.e., the greater the level, more users are expected to belong to that level. Some affiliate programs requires users to refer others according some specific criteria, like forcing a given number of referrals for each user, limiting the number of referrals by level and so on. Note that GPTEngine doesn't limit the way that members refer other users, so such type of forced matrix schemes can't be done with this software.
Users of GPT programs benefit from the referral system by earning a percentage of what their downline earns and/or by getting a bonus anytime a member is referred. A referral program is usually the most effective way of making money in a GPT website. The number of downline levels allowed in a GPT program should be carefully considered as it can deeply impact the program financially. The greater the number of downline levels, more difficult it becomes to keep the GPT website financially healthy. The recommended number of downline levels (as required by some payment processors like PayPal) is one.