A mixed lot splits conceptually into its round-lot portion, which can participate in the displayed quote, and its odd-lot remainder, which cannot. Execution is seamless and the fill report shows one position change, but the components can print separately on the tape.
This is mainly a reporting nuance. It matters when reconciling a fill against time-and-sales and finding the order appears as two prints at slightly different times.
Example: an order for 340 shares fills as 300 shares in three round-lot prints and a 40-share odd-lot print, four lines on the tape for one instruction.
Related: round-lot, odd-lot, fractional-shares, fill, time-and-sales