Beginner: Lotto

Lotto Definitions
Player TheOptician: To attack an unknown and unmoved piece with a non-expendable piece

Player Napoleon 1er: Lotto is to jump on an unknown unmoved piece with the goal to capture it when you are not forced to take risks. Jumping on an unknown unmoved piece with the goal to gain information is not lottoing. You are forced to take risks like jumping on an unknown unmoved piece that you believe might be a good capture for you when you are down in material or when if you do not take the risk and  initiative to change something in the game your opponent will with a high probability win due to his advantages.

Expendability / Appropriate Risk
From player TheOptician's Stratego.com forum post

If the player deems the piece to be expendable then it is the players opinion that she/he can afford to lose the piece without significantly damaging her/his chance of victory - or because the loss of that piece will provide information that mitigates the loss.

Some players have labelled it lotto to attack an undisturbed front row with a captain. Others don't have a problem using a major for this purpose. Either way the important distinction is whether the player believes the piece to be expendable or not

Circumstances in a particular game might lead to a player to decide that their highest pieces are expendable.

Luck Ratio
From player josephwhite's Stratego.com forum post

One component that is also important in factoring whether it is a reasonable lotto or not is the luck ratio. If the chances of hitting a bomb or moving next to a piece on level higher is 50% or more, then I would say that this is a lotto if the piece is not expendable. If the chance is much less due to various factors (e.g. 10%), then it may just be a reasonable calculated risk.

Therefore I would suggest that the two main factors are expendibility and luck probability. A low expendibility with low luck probability would be a lotto. There is obviously a large area of grey in between.

Need for luck
To win, it is often advantageous for a player who is playing someone with a much higher rank (e.g. 500 ELO points difference) to lotto at the beginning with high pieces to try to get a strong advantage over the stronger player. Once a reasonable advantage is gained, then the lotto can stop. Without lotto, the more skilled player will likely win every time.

Lotto effect on ELO (point system)
From player roeczak's Stratego.com forum post

You may win more games than you lose, but the way ELO works you will definitely lose more points because of lotto than you win.

Most of the players who lotto are bronze and silver (below 500 ELO points), which means for a player around 800 he has to win .... 19 out of 20 games with players of these ranks just to keep his rating. Of course we know that its not possible to have such a good score against lottoers: you'll score about 12-16 points over 20 games, and in the end you lose ELO.

This is why in my opinion you should only play opponents within 200-250 points of you rating. (Maybe 300-350 for players over 950). The game of Stratego itself is formed in a way that a player who just lottos randomly should be able to take at least 3 in 20 games from any player.

This argument also reinforces the benefit of lottoing for players who are of much lower rank than their opponent.