In the games
In Mega Man Battle Network 5 onwards, when Lan Hikari is operating a NetNavi other than MegaMan.EXE, he has the option of selecting a fixed Navi Data Chip. This chip can only be used once, and it is the NetNavi's most powerful attack (excluding Rockman.EXE 4.5 Real Operation)
With the release of the Battle Chip Gate products, Navi Data Chips were released to allow the player to play as the NetNavi represented in the chip throughout the game by slotting in any of these chips in the Battle Chip Gate.
It first appeared in the Japanese version of Mega Man Battle Network 4 as a Operation Battle secret mode that becomes available with the Battle Chip Gate, and as a major feature in Rockman.EXE 4.5 Real Operation. While the Battle Chip Gate wasn't released outside Japan, the mode is still present in the fourth game and can be unlocked in any version. Most Navi Data Chips were released in Operation Battle Starter boxes containing 8 Standard Chips, 1 Mega Chip, and 1 Navi Data Chip.
For Mega Man Battle Network 5, Navi Data Chips returned, but had a slightly different purpose. Slotting in a Navi Chip in the overworld allows the player to switch Navis for as long as the player wishes. Slotting in Navi Chips in the net or in a Net Battle would allow MegaMan to gain the Navi's Double Soul, even if the player hasn't unlocked that Navi's Soul yet. Interestingly, this will not change any conversation sequences in the game.