You are looking at configuration for WEP, the original and completely obsolete Wi-Fi encryption standard. It had a very simple system (no keying handshake, no KDF), just a raw static hexadecimal key for the RC4 encryption algorithm.
WEP devices could be programmed with four keys – one is selected as the "active" transmit (encryption) key, but all four work as receive (decryption) keys. So there's nothing more to it – "current transmit key" tells you which key is used when transmitting.
I think this feature might have been used for key rotation (if you program both the 'current' and 'next' keys, devices no longer need to switch at precisely the same time), and/or isolation between devices (some devices using key 1, others key 2, etc).
Note 1: The iwconfig and iwlist tools are built on WEXT, which modern Linux Wi-Fi drivers no longer use. Although the current nl80211/cfg80211 framework still provides emulated WEXT support, all the new features go into the nl80211-based iw tool instead. (There are some exceptions, e.g. a few Realtek drivers still use WEXT.)
Note 2: None of these tools deal with WPA at all – you won't be able to set your WPA passphrase through 'iwconfig' nor through 'iw'. Both WPA1 and WPA2 (RSN) use dynamic keying which is performed by wpa_supplicant or iwd daemons.
