What does =C in ENDFIL LDA =C'EOF' 032010 in SIC means

Question

I am trying to understand the concept literals in SIC machines.

I encountered a code snippet in the System Program by Leland Becks book

45 001A ENDFIL  LDA   =C'EOF'    032010

i am confused by what does C means here

it is explained as it specifies a 3-byte operand whose value is a character string EOF

is it because EOF is a character string we use C or is just a variable

Like wise

215   1062  WLOOP   TD  =X'05'   E32011

What does X means in this instruction too.

Any help would be highly appreciated.Thanks in advance