Draftsim AI uses the following concepts to evaluate each card:

Power:  

Each card has a base rating of 0-5.

Color:   

Cards that fit into a two-color deck are given a bonus.

A complete list of ratings can be found here.

Drafting

Stage 1: Speculation

At the beginning of a draft, the bots pick the highest rated card out of each pack. On-color cards are given a small bonus, which ranges from +0 to +.9.

Stage 2: Commitment

After the bots have taken some strong cards, they commit to drafting a two-color deck. A +2.0 bonus is given to on-color cards.

Stage 3: Deck Construction

During deck construction, automatically construct a deck with the most powerful 23 cards in two colors and 17 lands.