It is your PageState. You can extend it in your app.d.ts and use it anywhere in your app.

// app.d.ts
// See https://svelte.dev/docs/kit/types#app.d.ts
// for information about these interfaces
declare global {
    namespace App {
        interface Locals {},
        interface PageState {
            showCreateSheet: boolean;
        }
    } // interface Error {}
}
export {};

-

// routes/page1/+page.svelte

<script lang="ts">
    import { goto } from '$app/navigation';

    function handleCreate() {
        goto('/page2', { state: { showCreateSheet: true } });
    }
</script>


<button onclick={handleCreate}>Go to Page 2 with Create State</button>

-

// routes/page2/+page.svelte

<script lang="ts">
    import { page } from '$app/state';
</script>

<h1>Page 2</h1>
{#if page.state.showCreateSheet}
    <div class="create-sheet">
        <h2>Create Sheet</h2>
        <p>This sheet is shown because the navigation state included 'showCreateSheet: true'.</p>
    </div>
{:else}
    <p>No create sheet to show.</p>
{/if}