Script code will be executed both when the respondent moves forwards and backwards through the survey (except for validation code scripts - validation code is only executed when going forwards in the questionnaire).
This is important to bear in mind if your survey has a Back button. If for example you include a SendMail script, you might not want an email to be triggered both when the respondent goes forwards and backwards. If your scripts are used to set hidden variables based on certain conditions, you may also consider whether you need to remove the values that were set when going backwards, instead of resetting them. There is a function called Forward()(go to Forward for more information) which can be used for this purpose, to distinguish between actions taken when respondents move forwards and backwards.