The behavioural strategies assign, independently for each information set, a probability distribution over actions.
The sequence form game is a game representation that, given a node W, a sequence q of player i is the ordered set of actions of player i appearing in the path from the root of the game tree to W. The strategy that you can define on the sequence form is called realization strategy.
I would like to have some intuitive explanation of why these kind of strategies and game representations are ill-defined in the imperfect recall set up.