Does `#[account(init)]` create a PDA account or a non-PDA account?

Question

1. Does the below declaration mean to create a PDA account?

#[derive(Accounts)]
pub struct CreatePixel<'info> {
    #[account(init, payer=user, space=Pixel::LEN)]
    pub pixel: Account<'info, Pixel>,
    #[account(mut)]
    pub user: Signer<'info>,
    pub system_program: Program<'info, System>,
}

2. Or must I use 'seeds' to create PDA a account？ No matter the seeds' value, is even "" (empty) acceptable?

// Data validators
#[derive(Accounts)]
pub struct CreateCounter<'info> {
    #[account(mut)]
    authority: Signer<'info>,
    #[account(
        init,
        seeds = [],
        bump,
        payer = authority,
        space = 100
    )]
    counter: Account<'info, Counter>,
    system_program: Program<'info, System>,
}

Answer 1

Does bellow declare mean to create a PDA account?

pub struct CreatePixel<'info> {
   #[account(init, payer=user, space=Pixel::LEN)]
   pub pixel: Account<'info, Pixel>,
   #[account(mut)]
   pub user: Signer<'info>,
   pub system_program: Program<'info, System>,
}

No, the init constraint without seeds and bump doesn't create the PDA account. It will mark the account as a signer and expect you to provide the signature while calling the instruction.

or must I use 'seeds' to create PDA account？ no matter the seeds value, even "" empty is acceptable?

This is correct! You must use both seeds and bump constraints to initialize a PDA account. Seed is a byte-array so empty array is also a valid seed.