SCORE must follow sandbox policy, i.e. some operations like file system access or network API calls are strictly prohibited. A transaction sent from a user will be executed on every node that exists on the ICON network. And the nodes will form consensus on the transaction result. So deterministic behavior is very important aspect when you write SCORE code, because every node should agree on the execution result. Therefore you cannot use a random operation like using random() function, cannot make network call that may get different results depending on the connection, cannot make system call that results may also differ between nodes. Also, you cannot do long-running operations in SCORE, that may prevent forming consensus between nodes. These sandbox policies should be obeyed by every SCORE programmer.