EncoderId
One encoder, by its FFmpeg name (libx264, h264_videotoolbox, aac): an implementation that produces a CodecId. FFmpeg.codecOf says which format, and FFmpeg.encodersFor lists the encoders a build has for a format.
Which encoders exist depends on how FFmpeg was built. The published artifacts are LGPL and carry no libx264 or libx265; the software video encoder every build has is Mpeg4.