The export format is .dat? That sounds like a Windows OS file type. Why not use JSON the way ACF does? Everybody is familiar with JSON, there is such universal support for it, and there is lots of option to exchange JSON/PHP, a dev might for instance create some automation to edit their JSON definitions if their the same field group across multiple sites and need to update them. Is the .dat format workable in this manner?
Previously, we use serialized data to store the data (just like WP do for all the options and post/term/user meta). So we use the .dat extension, which simply means "data". You can convert the data between PHP/.dat easily using serialize/unserialize functions. There's no difference between that and json_encode/json_decode.