QSFP28 transceivers is designed for 100G applications. Note that, QSFP28 can do 4x25G and 2x50G breakout connection, or 1x100G depending on the transceiver type that is used. The are two types of QSFP28 connector, LC Duplex or MPO/MTP, checking the QSFP28 specification for a best practice.
Some HPE switch provides QSFP28 port operates as 40GE ports by default. One thing to note is that you can't put a QSFP+ transceiver into QSFP28 ports to avoid destroying your optics.