When you place a market order by dollar amount, the system allocates the dollar value you entered. However, the quantity shown in Order Details is rounded to 4 decimal places.

Example:

You purchase $100 of stock X priced at $2,978.46. The actual quantity purchased is 0.181379573 shares, but for display purposes, the app rounds this to 0.1813 shares.

Rest assured that the full $100 is fully invested. Fractional shares are calculated accurately in the backend and correctly allocated to your account. The rounding in the app is only for display simplicity.