Symbol vs. Word (Digital Communication Theory)
A symbol is what is transmitted ,  received and detected and there are many different symbols for a bit, a sync word and perhaps others in different forms of communication.
When frequency compression is used such as QAM/QPSK there can be many bits per symbol or bits/Hz BW.  In RZ baseband a symbol is a pulse that Returns to Zero if it is a &quot;1&quot; so the &quot;symbol&quot; is 1/2 bit also same for Manchester or Bi-Phase the symbols are twice the bit rate.
In PCM it is binary with 1 bit per symbol but it is also synchronous so there is a special unique sync word for frame synchronization so here the symbol is the Sync word or I guess it could be called a Binary Word Assignment.  There may be other words for Forward Error Correction such as Hamming Codes.  Each of these are also Symbols but used synchronized to the frame and used to improve symbol error rate , frame synchronization and thus resulting bit error rate (BER) and  message error rate for a block of words.
So Sync words can also be &quot;symbols&quot; which express a certain pattern and may also satisfy other theoetical opitmization such as Correlation Function in case of low SNR and improvements on Sync Symbol detection with an auto-correlator.
Symbol definition in a general sense. Ref: Wiki

Symbols take the form of words, sounds, gestures, ideas or visual images and are used to convey other ideas and beliefs.
For example, a red octagon may be a symbol for &quot;STOP&quot;,

... which is an english traffic word, as is each character and so is each bit and sync word in PCM.
However data words are not classified as symbols as they can be random data which may be assigned to specific meanings not generally defined as &quot;symbols&quot; in communication but perhaps values in  channels such  part of a DS1 Telephony data frame structure or diagnostic signalling S channel.