See Simone Villa's post of 164 days ago (last but one)
Advanced Strategy in a nutshell
This is the pertinent bit:
2) advanced 1 strategy after race start
- it decides which kind of tyre you will wear at next pit stop if water reaches decided level (it works only if advanced strategy are ACTIVATED)
-it decides which kind of tyre you will wear as begins to rain (it works only if advanced stategy is DEACTIVATED or you do not make strategy )