They informally and incorrectly but invariably use the name "Simplex" when referring to the divisional tactical channels, which are officially named "Enhanced/Fallback" frequencies, channels 101-125. When using them, the officers can select either direct car-to-car (true "simplex") mode or "duplex" repeater mode. It's not at all uncommon for one unit to be TXing on simplex and the other on duplex - they're both still hearing the lower "downlink" side of the pair. These Enhanced/Fallback repeaters are located at each division station, so they don't have the range of the bureau and citywide tactical frequencies.
The eight Bureau and seven Citywide tac frequencies, channels 36-49 and 177, are also officer-selectable for either simplex or duplex mode and those repeaters are located primarily on mountaintops around the city for much greater range.
In the UHF analog years, 1981-2001, they routinely used their divisional "Base" (dispatch) frequency in simplex mode to talk car-to-car, but when digital came along in June 2001 they very quickly discovered that digital's capture-or-kill effect caused too much interference to the dispatchers' transmissions. So the dispatch Base frequencies (channels 1-25) are now capable of duplex mode only, and officers very seldom talk to each other on them except during emergent situations until they are given a tactical frequency to switch to.
