The minecraft:passing sequence handler continues if and only if the previous action executed successfully, unless that action was optional. The sequence handler is successful if all actions were either executed successfully or were optional.

{
  "type": "minecraft:sequence",
  "handler": "minecraft:passing",
  "entries": [
    {
      "action": {
        "type": "minecraft:modify_block_state",
        "position": "target",
        "properties": {
          "eye": "true"
        },
        "push_entities_upwards": true
      }
    },
    {
      "entry": {
        "action": {
          "type": "minecraft:light_end_portal",
          "position": "target"
        }
      },
      "optional": true
    }
  ]
}

An entry that may be optional. If the entry is optional, it is always considered successful, regardless of its actual result. If it is not, the entry may also be inlined as a normal action entry.

{
  "entry": {
    "action": {
      "type": "minecraft:light_end_portal",
      "position": "target"
    }
  },
  "optional": true
}

{
  "action": {
    "type": "minecraft:apply_suspicious_stew_effects_from_item",
    "entity": "this"
  }
}